A  TEXT-BOOK 


OF 


PLANT  PHYSIOLOGY 


BY 


GEORGE  JAMES  PEIRCE,  Ph.D. 

Associate    Professor    of  Plant    Physiology 
Leland  Stanford  Junior  University 


NEW    YORK 

HENRY   HOLT  AND   COMPANY 
1903 


Copyright,  1903 

BY 
HENRY  HOLT  &  Co. 


~     o-  THE 
UNIVERSITY 

^ 


PREFACE 

The  course  in  Plant  Physiology  which  I  have  given  for  the 
last  five  years  at  Stanford  University  has  impressed  upon 
me  the  need  of  a  text-book  which  treats  the  subject  less  ex- 
haustively  than  Pfeffer's  Handbuch  and  more  fully  than 
Noll's  section  of  the  Bonn  text-book.*  As  I  did  not  know 
of  such  a  book  in  any  language,  I  began  to  write  my  lec- 
tures. These  lectures,  after  repeated  working  over,  have 
finally  taken  this  definite  form. 

My  intention  has  been  to  present  the  main  facts  of  plant- 
physiology  and  the  saner  hypotheses  regarding  them,  striv- 
ing to  express  safe  views  rather  than  to  echo  the  most  re- 
cent, attempting  here  and  there  to  suggest  definite  problems 
for  investigation,  and  everywhere  trying  to  avoid  giving  the 
impression  that  the  science  or  any  part  of  it  has  reached 
ultimate  knowledge  and  final  conclusions. 

I  have  purposely  made  no  attempt  to  give  directions  for 
experiments,  believing  that  a  laboratory  manual  and  a  text- 
book must  meet  such  different  needs  that  the  style  of  the 
one  is  impossible  for  the  other. .  This  book  must,  however, 
be  supplemented  by  actual  laboratory  work  by  the  reader, 
under  the  guidance  of  a  teacher  or  of  some  of  the  laboratory 
manuals  mentioned  on  page  27,  or  both. 

No  one  can  work  on  the  physiology  of  plants  nowadays 
without  being  conscious  of  his  indebtedness  to  Pfeffer.  If 
Pfeffer  had  done  nothing  else,  the  preparation  of  the  two 
editions  of  his  Handbuch,  in  which  the  literature  of  the  sub- 
ject is  brought  together,  and  the  present  status  of  the  sci- 
ence is  well  set  forth,  would  secure  him  an  honored  place.  I 
cannot  refrain  from  acknowledging  my  personal  as  well  as 

*Lehrbuch  der  Botanik.  Strasburger,  Noll,  Schenck,  Schimper.  Five 
editions.  English  translation  by  Porter. 


iv  PREFACE 

professional  obligation  to  him,  and  I  do  so  with  the  utmost 
satisfaction. 

It  is  also  a  great  pleasure  to  express  my  grateful  apprecia- 
tion of  the  help  I  have  received  from  my  associates  in  this 
University,  Dr.  Anstruther  A.  Lawson  (Botany),  Professors 
Herman  De  C.  Stearns  (Physics),  Frank  M.  McFarland 
(Histology),  William  A.  Cooper  (German),  and  especially 
from  Professor  Douglas  H.  Campbell.  I  am  indebted  also 
in  various  ways  to  my  friends  Professor  William  F.  Ganong 
of  Smith  College,  and  Dr.  Hermann  von  Schrenck  of  Wash- 
ington University,  St.  Louis.  Professor  Walter  R.  Shaw  of 
the  University  of  Oklahoma  generously  allowed  me  to  use 
his  excellent  photograph  of  Postelsias  reproduced  on  page 
190.  Dr.  Daniel  T.  MacDougal  of  the  New  York  Botanical 
Garden  and  his  publishers,  Messrs.  Longmans,  Green  &  Co., 
have  given  me  permission  to  use  the  figures  of  Mimosa  on 
pages  248-9.  Professor  Goebel  of  Munich  kindly  sanctions 
my  use  of  his  figure  of  Opuntia  shown  on  page  210.  To  these 
and  all  others  who  have  assisted  me  I  wish  most  heartily  to 
express  my  sincerest  thanks. 

G.  J.  P. 

Leland  Stanford  Junior  University, 
California,  December,  1902. 


CONTENTS. 


CHAPTER  I.  INTRODUCTION. 

Introduction 1 

The  Conditions  Essential  to  Life 6 

The  Living  Matter  and  the  Actively  Living  Structure 7 

CHAPTER  II.  RESPIRATION. 

Respiration 12 

Intramolecular  Respiration 27 

Fermentations 30 

CHAPTER  III.  NUTRITION. 

Nutrition 40 

The  Food-materials 42 

Carbon 43 

Chlorophyll 51 

Photosynthesis 58 

Nitrogen 66 

Root-tubercle  Plants 72 

Humus  Plants 78 

Carnivorous  Plants 81 

Parasites 85 

Ash-constituents 92 

CHAPTER  IV.  ABSORPTION  AND  MOVEMENT  OP    WATER.     FOOD  DIS- 
TRIBUTION. 

Absorption ' . . .  103 

Diffusion  and  Osmosis 108 

Means  of  Absorption  of  Nutrient  Solutions 113 

Means  of  Transfer  of  Nutrient  Solutions 116 

Secretion 125 

Sap-pressure  and  Bleeding 130 

Transpiration 136 

Stomata  and  the  Aerating  System 142 

Gases  and  Movements  of  Gases 151 

Translocation  of  Foods 155 

CHAPTER  V.  GROWTH. 

Growth 162 

Periodicity  of  Growth 169 

Relations  of  Growth  and  Turgor 172 

Growth  Pressures 174 

Rate  of  Growth 176 

Limit  of  Growth 179 

V 


VI  CONTENTS. 

PAGE 

CHAPTER  VI.  IRRITABILITY. 

Irritability 183 

Physical  Basis  of  Irritability 184 

Irritability  and  the  Amount  and  Kind  of  Growth 186 

Influence  of  Gravitation 196 

Influence  of  Light 208 

Influence  of  Heat 219 

Influence  of  Water 222 

Influence  of  Other  Substances 226 

Influence  of  Electricity 237 

Influence  of  Contact 239 

Conclusion 251 

CHAPTER  VII.  REPRODUCTION. 

Reproduction. 254 

Heredity 279 

INDEX..                                                                                                      .  285 


f 


PLANT  PHYSIOLOGY 


CHAPTER   I 
INTRODUCTION 

ALL  living  beings  are  alike  in  kind,  differing  from  one  an- 
other only  in  degree.  To  this  conclusion  scientific  men  have 
been  gradually  driven  back  by  the  failure  of  every  attempt 
to  discover  and  to  define  any  fundamental  difference  between 
animals  and  plants.  The  differences  between  the  higher  rep- 
resentatives of  the  two  so-called  kingdoms— animal  and  veg- 
etable— are  so  striking  that  no  one  can  fail  to  see  them. 
Between  the  higher  and  the  lower  animal  forms  are  differ- 
ences as  striking  as  those  between  higher  animals  and  higher 
plants.  Between  the  higher  and  lower  plant  forms  there  are 
similar  differences.  Likenesses  always  accompany  these  dif- 
ferences. It  is  these  likenesses  which  enable  us  to  call  a 
given  organism  an  animal  or  a  plant.  Between  the  higher 
and  lower  members  of  the  same  kingdom,  and  between  the 
higher  members  of  the  two  kingdoms,  the  differences  are  so 
striking  that  the  attention  is  almost  wholly  occupied  with 
them.  It  is  natural  that  we  should  look  more  for  differences 
than  for  likenesses :  for  our  ability  to  distinguish  our  friends 
from  our  enemies,  our  own  from  our  neighbor's  possessions, 
the  Bird-foot  Violet  from  the  Swamp  Violet,  the  dog  from 
the  tree,  is  dependent  upon  our  perception  of  differences 
between  them.  It  is  necessary  that  we  see  differences :  the 
more  intelligent  the  man,  the  keener  is  his  perception  of 
differences;  the  higher  the  organism,  the  greater  is  its 
difference  from  others. 

The  study  of  low  organisms  reveals  few  differences.  Suc- 
cessfully to  study  such  low  organisms  as  the  bacteria  de- 


2  PLANT  PHYSIOLOGY 

mands  unusual  ability  to  detect  the  differences  between 
them.  The  differences  disappear  as  we  descend  the  scale  of 
development,  and  the  likenesses  become  more  and  more  evi- 
dent. The  bacteria  and  diatoms,  which  have  been  repeat- 
edly regarded  first  as  animals,  then  as  plants,  and  the 
Myxomycetes  (slime-moulds),  which  are  still  believed  by 
some  to  be  animals  ( Mycetozoa ) ,  illustrate  the  difficulty 
of  determining  to  which  "  kingdom"  these  organisms  belong. 
It  is  important  to  the  students  of  systematic  botany  and 
zoology  to  know  where  to  place  these  organisms  in  their 
systems  of  classification.  This  enables  other  naturalists  to 
go  further.  To  the  physiologist  it  is  a  convenience  rather 
than  an  essential  to  know  whether  the  organism  which  he 
is  studying  is  called  an  animal  or  a  plant,  but  the  results 
of  his  work  have  often  been  useful  to  the  systematists  in 
confirming  or  correcting  their  classifications.  From  the 
classifier's  point  of  view  the  differences  which  enable  him 
to  make  an  orderly  arrangement  of  the  objects  of  his  study 
are  of  the  utmost  importance;  from  the  physiologist's 
standpoint  the  likenesses  are  most  important.  This  will  be- 
come plainer  when  we  state  the  aim  of  physiology. 

THE  AIM  OF  PHYSIOLOGY 

According  to  Pfeffer,*  "the  aim  of  physiology  is  to  study 
the  nature  of  all  vital  phenomena  in  such  a  manner  that, 
by  referring  them  to  their  immediate  causes,  and  subse- 
quently tracing  them  to  their  ultimate  origin,  we  may  ar- 
rive at  a  complete  knowledge  of  their  importance  in  the  life 
of  the  organism."  Physiology  is  a  study  not  merely  of 
structure,  though  to  its  successful  pursuit  a  knowledge  of 
structure  is  indispensable ;  nor  of  organized  bodies,  though 
a  knowledge  of  the  laws  which  govern  their  organization 
( structure  and  form )  is  important.  It  is  the  study  of  the 
living  organism.  Crystals  have  definite  and  characteristic 
structures  and  forms,  they  increase  in  size  and  in  number  in 
accordance  with  a  few  laws  common  to  all  and  a  few  laws 

*  Handbuch  der  Pflanzenphysiologie,  2te  Auflage,  Bd.  I.,  p.  7,  1897. 
English  translation  by  Ewart,  Physiology  of  Plants,  vol.  I.,  p.  8,  1900. 


INTRODUCTION  3 

peculiar  to  each  kind.  The  carbon  compounds  have  definite 
structures,  as  Kekule's  demonstration  of  the  benzole-ring 
proved;  they  form  under  certain  conditions  and  their  be- 
havior is  characteristic  of  the  kind.  A  machine  has  defi- 
nite structure,  it  operates  in  a  fashion  characteristic  of  its 
kind.  Physiology  has  long  been  conceived  to  be  the  study 
of  the  structure  and  operations  of  peculiar  machines,  the 
study  of  functions  as  based  upon  a  knowledge  of  anatomy. 
The  physiologist  is  now  striving  not  only  to  know  the  func- 
tions which  are  the  manifestations  of  the  life  possessed  by 
complicated  living  structures  or  organisms,  but  also  to 
determine  the  causes  both  of  structure  and  of  functions.  In 
an  engine  we  have  a  structure  which,  under  certain  know- 
able  conditions,  does  certain  kinds  of  work.  The  materials 
and  the  construction  of  the  engine  are  lifeless;  the  engine 
is  the  result  of  human  intelligence  acting  in  harmony 
with  physical  laws  upon  lifeless  material;  the  working  of 
the  engine  is  the  result  of  energy  (physical  force)  applied 
through  human  intelligence  to  it ;  the  structure  itself  and  its 
working  are  the  result  of  physical  forces  acting  upon  inert 
materials  in  harmony  with  physical  laws  comprehended  by 
the  designer  and  driver  of  the  engine.  These  are  all  exter- 
nal to  the  structure  and  are  consciously  taken  advantage  of 
and  applied  to  and  through  the  engine  by  the  living  organ- 
isms concerned  in  its  construction  and  operation. 

A  living  organism  is  a  structure  existing  in  harmony  with 
physical  forces  and  laws  and  because  of  them.  Few  living 
organisms  strive  to  ascertain,  and  none  fully  knows,  what 
these  forces  are.  The  materials  employed  in  the  construc- 
tion of  a  living  organism  are  inert,  lifeless;  they  are  ar- 
ranged in  harmony  with,  and  through  the  operation  of, 
physical  laws  and  forces ;  but  the  inert  materials  are  acted 
upon  and  the  physical  forces  employed  by  the  organism 
itself,  and  for  the  most  part  unconsciously. 

Between  the  engine  and  the  living  organism  there  is  then 
a  radical  difference;  the  engine  and  the  organism,  though 
both  machines,  differ  in  that  life,  external  though  applied 
to  the  one,  is  internal  and  possessed  by  the  other.  The 
difference  thus  stated  is  one  plainly  felt  but  inadequately 


4  PLANT  PHYSIOLOGY 

expressed,  for  who  can  tell  what  life  is?  Just  as  we  say 
that  walking  consists  in  the  rapid  restoration  of  the  body 
to  a  position  of  equilibrium  after  falling,  so  we  may  say  that 
living  consists  in  the  maintenance  of  the  equilibrium  between 
constructive  and  destructive  influences  and  processes.  But 
living  is  the  evidence  of  life,  just  as  thinking  is  the  evidence 
of  brain ;  living  is  not  life  itself.  However,  in  studying  the 
means  by  which  the  equilibrium  between  constructive  and 
destructive  influences  is  maintained— the  means  by  which 
living  is  attained — we  are  approaching  the  eternal  question  : 
What  is  life  itself? 

In  determining  that  the  means  of  maintaining  the  equi- 
librium between  constructive  and  destructive  influences  are 
physical  and  chemical,  and  that  the  influences  themselves 
are  physical  and  chemical,  not  peculiar  "  vital"  forces,  not 
occult  or  supernatural,  •  one  question  regarding  life  is  an- 
swered. Whatever  may  be  our  views  regarding  the  origin  of 
life,  there  is  no  scientific  or  other  heresy  in  accepting  the 
present  evidence  that  life  maintains  itself,  and  is  maintained, 
by  physical  and  chemical  means  only.  In  the  following 
pages  these  means  will  be  examined  and  discussed. 

Pfeffer's  statement,  quoted  above,  of  the  aim  of  physiology 
does  not  limit  the  study  to  the  manifestations  of  life  in 
either  "kingdom"  of  organisms.  One  evidence  of  the  wis- 
dom of  such  a  broad  view  is  the  value  which  Pfeffer's  own 
investigations,  conducted  mainly  on  plants— particularly 
perhaps  those  on  osmosis  and  on  irritability— possess  for 
the  animal  physiologist.  When  physiologists,  not  satisfied 
with  the  observation  and  description  of  the  phenomena  dis- 
played by  the  different  kinds  of  living  organisms,  began  to 
seek  for  the  means  by  which  these  phenomena  are  executed 
and  for  their  causes,  comparison  revealed  that  the  causes, 
the  means,  and  even  the  phenomena  themselves,  are  alike 
in  all  organisms.  Physiology  is  now,  therefore,  a  much 
broader  as  well  as  deeper  science  than  it  was  formerly  con- 
ceived to  be,  and  though  there  are  now  and  always  must  be 
animal  and  plant  physiologists,  they  are  studying  common 
problems  and  contributing  to  the  common  mass  of  human 
knowledge  of  living  organisms. 


INTRODUCTION  5 

It  is  often  a  matter  of  chance  or  of  convenience  which 
determines  whether  a  man  shall  study  one  organism  or  an- 
other. Sachs,*  who  may  be  called  the  founder  of  modern 
plant-physiology,  has  said  that  although  plant-physiology 
owes  much  to  animal-physiology,  yet  animal-physiology  is 
being  enriched  by  the  results  attained  by  plant-physiolo- 
gists. The  appreciation  of  the  fundamental  unity  of  the 
aims  and  the  results  of  animal  and  plant  physiologists  has 
recently  led  to  the  publication  of  two  books  on  general 
physiology,  f  In  such  books  the  manifestations  of  life  are 
described  and  discussed  in  a  broad  way;  for  details  one 
must  turn  to  the  special  treatises  on  the  physiology  of 
animals  and  plants. 

Though  the  phenomena  of  life  are  the  same  in  all  organ- 
isms, life  manifests  itself  in  special  ways  in  different  or- 
ganisms, making  some  more  favorable  for  the  study  of 
special  phenomena  than  others.  For  example,  the  manu- 
facture of  starch  from  carbon-dioxide  and  water,  and  the 
formation  of  tannin  and  certain  other  by-products  in  nutri- 
tion, are  subjects  in  plant-physiology  only,  just  as  other 
special  functions,  such  as  those  of  nerves  and  muscles,  car- 
ried out  by  extremely  differentiated  organs,  and  the  circu- 
lation in  vessels,  are  subjects  in  animal-physiology  only. 
Again,  it  is  more  convenient  to  study  on  low,  small,  aquatic 
plants  and  animals  some  of  the  effects  of  light  than  on 
higher  terrestrial  organisms,  but  the  comparison  of  higher 
and  more  complex  forms  with  the  lower  shows  that  the 
effects  are  identical  in  kind  if  not  in  degree.  For  this  rea- 
son, and  because  no  "one  man  can  know  all  the  parts  of  the 
whole  subject  of  physiology  equally  well,  it  must  still  be 
divided.  While  the  following  pages  will  be  devoted  mainly 
to  the  study  of  plants,  the  reader  should  bear  constantly  in 
mind  that,  at  the  same  time  that  there  are  special  vital 

*  Sachs,  J.  von.  Lectures  on  the  physiology  of  plants,  English  transla- 
tion, by  H.  M.  Ward,  p.  650.  Oxford,  1887. 

t  Verworn,  M.  Allgemeine  Physiologic,  two  editions.  English  transla- 
tion, by  Lee.  General  Physiology,  New  York,  1899.  Davenport,  C.  B. 
Experimental.  Morphology.  Parts  I.  and  II.,  New  York,  1897, 1899.  Others 
to  follow. 


6  PLANT  PHYSIOLOGY 

activities — special  manifestations  of  life — according  to  species 
and  individuals  as  well  as  "kingdoms/'  there  are  also 
general  vital  activities  common  to  all  living  things.  Before 
passing  to  any  consideration  of  these  in  detail,  it  will  be 
well  to  enumerate  the  conditions  essential  to  life,  to  con- 
sider the  material  and  the  structure  in  which  life  is  resident 
and  which  manifest  it,  and  to  realize  that  these  conditions, 
this  material  and  structure,  are  the  same  for  all  living* 
things. 

I.     THE  CONDITIONS  ESSENTIAL  TO  LIFE 

These  may  be  comprehended  under  five  headings:  — 

1.  Proper  Food— (a)  the  source  of  the  materials  of  which 

the  body  is  built,  and 

(b)    of  the  energy  by  which  the  body  is  built    and 
operated. 

2.  Water — (a)  the  vehicle  of  the  food-materials  and  of  the 

foods  absorbed  into  the  body  and  transferred  from 
part  to  part,  and  also 

(b)     an    indispensable  component  of  actively  living 
protoplasm. 

3.  Proper  Temperature— which  makes  possible  the  vital,  i.e. 

the  chemical  and  physical,  changes  which  must  go 
on  within  the  body,  and  in  all  of  its  parts,  lest  in- 
action and  death  ensue. 

4.  Proper  Illumination— which  furnishes  the  organism  with 

the  forms  of  energy— physical  and  chemical— thermal, 
luminous,  and  actinic — of  which  it  is  directly  or  in- 
directly in  need. 

5.  Proper  Freedom — freedom  from  mechanical    and    other 

disturbances  which  would  interfere  with  its  supply  of 
food,  water,  warmth,  and  light,  and  prevent  it  from 
carrying  on  its  natural  functions. 
(To  this  list  some  might  wish  to  add  Oxygen,   but 
this  is  included  under  the  first  heading. ) 

The  many  forces  and  matters  included  in  this  brief  sum- 
mary are  not  merely  passively  essential  to  life ;  they  actively 


INTRODUCTION  7 

stimulate  all  living  organisms.  The  organisms  are  sensitive, 
and  respond  to  these  stimuli.  Supplying  food  to  animal  or 
plant  is  applying  a  stimulus,  as  well  as  providing  the  means 
to  further  and  continued  action.  The  response  to  the  stimu- 
lus may  vary  with  the  organism,  be  immediate  or  delayed, 
be  external  and  visible  to  the  eye,  or  merely  internal;  but 
every  organism  lives,  both  because  the  conditions  make  liv- 
ing possible,  and  also  in  accordance  with  and  in  response  to 
the  many  and  diverse  stimuli  exerted  upon  it  by  these  con- 
ditions. The  different  reactions  of  different  organisms  to 
the  same  stimulus  do  not  imply  any  special  or  peculiar 
vital  force ;  on  the  contrary,  they  imply  the  special  or  pecul- 
iar structure  of  these  different  organisms.  The  force  of 
gravitation  pulls  some  things  down,  but  it  is  the  same 
force  which  keeps  other  things  up.  The  Eiffel  Tower,  con- 
structed in  opposition  to  the  force  of  gravitation,  stands 
now  because  of  it.  So  all  living  organisms,  subjected  to 
like  forces  and  supplied  with  like  materials,  behave  accord- 
ing to  the  characteristic  habits  of  each  species  and  the 
peculiar  habits  of  each  individual. 

II.     THE  LIVING  MATTER  AND  THE  ACTIVELY  LIVING 
STRUCTURE 

As  Hertwig  has  so  strongly  emphasized,*  the  living  and 
active  protoplasm  is  to  be  regarded  not  as  a  chemical 
compound  or  an  association  of  chemical  compounds,  but 
rather  as  an  orderly  arrangement  of  these  into  a  definite 
structure,  of  which  water  is  an  indispensable  constituent. 
Some  of  the  water  contained  within  the  cell  should  be  con- 
sidered to  be  as  much  a  constructive  constituent  of  the  liv- 
ing protoplast  as  the  water  is  of  the  crystal  of  copper 
sulphate.  As,  without  a  certain  amount  of  water,  one  can 
never  have  crystals,  no  matter  how  much  copper  sulphate 
may  be  present,  so  also,  without  the  necessary  amount  of 
water  we  can  never  have  active  protoplasm.  When  the 
water  of  constitution  is  withdrawn,  all  the  activities  of  the 

*  Hertwig,  Oscar.  Die  Zelle  und  die  Gewebe,  Bd.  I.,  p.  15.  The  Cell— 
translation  by  Campbell,  vol.  I. 


8  PLANT  PHYSIOLOGY 

cell  cease  with  the  demolition  of  its  structure.  Dehydration 
is  fatal  to  nearly  all  plants  and  their  parts,  rapid  dehydra- 
tion is  fatal  to  all;  but  ripe  seeds,  in  which  all  the  vital 
activities  are  greatly  reduced  as  the  embryo  attains  the 
stage  of  development  characteristic  of  the  species,  can  with- 
out injury  slowly  give  up  nearly  all  the  water  which  they 
contain.  By  this  means  the  protoplasmic  structure,  the 
arrangement  of  the  protoplasmic  particles,  necessary  to 
active  life,  is  sufficiently  altered  to  cause  an  entire  suspen- 
sion of  all  activities.  In  a  climate  of  average  humidity 
water  will  still  remain  in  air-dry  and  wholly  dormant 
seeds,  as  may  be  shown  by  weighing  them  air-dry  and  re- 
weighing  after  they  have  been  dried  for  an  hour  in  an  oven 
at  a  temperature  of  70°  C.,  or  for  a  longer  time  at  room 
temperature  in  a  desiccator.  The  following  table,  quoted 
from  Schroder,*  shows  the  percentage  cf  water  contained 
in  ripe  and  air-dry  grass  " seeds,"  thus — 

Hordeum  vulgare,  14.65% 
Triticum  durum,  14.63% 
Triticum  spelta,  14.40% 

This  amount,  though  considerable,  is  less  than  the  con- 
stitutional water  of  the  active  protoplasmic  structure,  and 
hence  all  vital  activity  is  extremely  slight.  With  the  restora- 
tion of  the  water  of  constitution,  and  after  the  lapse  of  a 
part  if  not  all  of  the  usual  "resting  period'7  of  the  seeds, 
the  vital  functions  will  be  resumed. 

The  spores  of  many  of  the  lower  plants  are  also  able  to 
bear  the  withdrawal  of  the  water  of  constitution  without 
permanent  injury,  and  like  seeds,  they  can  withstand  during 
this  period  of  inaction  degrees  of  heat  and  cold,  amounts 
of  poisonous  gases,  and  other  influences  which  at  other 
times  would  be  fatal  to  them.  The  dry  seeds  of  peas  and 
beans,  the  grains,  etc.,  and  the  dry  spores  of  various  fungi, 
can  be  subjected  for  hours  to  a  temperature  of  about  100° 
C.  without  destroying  their  germinating  power,  although 
about  half  that  temperature  would  in  fifteen  minutes  be 

*  Untersuchungen  aus  dem  Botan.    Institut  zu  Tubingen,  Bd.  II.,  p.  10, 

1886. 


INTRODUCTION  9 

fatal  to  them  after  germination.  The  active  cells  of  the 
great  majority  of  bacteria  will  be  killed  in  ten  minutes 
by  a  temperature  of  50  to  60  C.,  or  in  five  minutes  by  a 
temperature  of  70°  C.,  although  the  dry  spores  of  Bacillus 
anthracis  succumb  only  after  heating  for  three  hours  at 
140°  C.* 

During  the  period  of  nearly  complete  dryness  seeds  and 
spores  are  still  alive,  but  the  evidences  of  life  are  extremely 
difficult  to  detect.  Respiration  goes  on  very  feebly  indeed 
in  air-dry  seeds,  yet  that  these  seeds  do  respire  is  claimed 
by  Kolkwitz  f  as  the  result  of  refined  methods  of  collecting 
and  measuring  small  quantities  of  carbon-dioxide;  but 
seeds  containing  still  less  water  respire  even  less,  and  with 
no  constitutional  water  all  respiration  ceases. 

Since  water  is  in  itself  lifeless,  its  presence  or  absence  is 
merely  a  condition  which  makes  life  possible  or  the  reverse. 
Life  may  be  resident  in  some  other  of  the  chemical  com- 
pounds composing  protoplasm,  but  it  will  manifest  itself 
only  when  wrater  is  present  in  sufficient  amount.  Although 
the  ability  to  form  crystals  of  the  characteristic  size,  form, 
and  color  resides  in  molecules  of  copper  sulphate  and  not  in 
molecules  of  water,  no  copper  sulphate  crystals  will  form 
until  CuS04  molecules  are  accompanied  by  a  sufficient  num- 
ber of  water  molecules  to  make  the  crystalline  structure  a 
physical  possibility.  Another  comparison,  if  not  pressed 
too  far,  may  also  assist  in  emphasizing  and  elucidating 
this  matter.  The  living  protoplasm  deprived  of  water  may 
be  likened  to  the  disconnected  parts  of  a  machine;  it  may 
be  heated  or  chilled  or  subjected  to  other  kinds  of  harsh 
treatment  without  greatly,  if  at  all,  affecting  it;  but  set 
up  the  machine,  furnish  it  with  energy,  and  it  will  work- 
give  an  abundance  of  water  to  the  protoplasm  so  that 
it  may  set  itself  up  into  a  machine,  allow  it  to  furnish 
itself  with  energy,  and  it  will  work.  But  though  water 
is  indispensable  to  the  carrying  on  of  the  vital  func- 

*  Fischer.  Vorlesungen  uber  Bakterein,  Jena,  -1897,  p.  72.  English  trans, 
by  Jones,  Structure  and  functions  of  bacteria.  Oxford,  1900,  p.  76. 

t  Kolkwitz,  R.  f'ber  die  Athmung  ruhender  Samen.  Ber.  d.  D.  Bot* 
Ges.,  Bd.  XIX.,  1901. 


10  PLANT  PHYSIOLOGY 

tions,  it  is  not  in  all  cases  essential  to  the  preservation 
of  life. 

The  vital  functions  may  be  suspended,  without  deterio- 
ration of  the  germinating  power  of  the  resting  body,  for 
periods  varying  with  the  species  and  in  some  instances 
astonishingly  long.  As  a  matter  of  safety,  the  dry  seed  or 
spore  should  regularly  retain  its  vitality  at  least  for  some- 
what longer  than  the  usual  period  during  which,  by  reason 
of  heat,  or  cold,  or  drought,  active  life  is  impossible.  Some 
seeds  may  retain  their  germinating  power  for  much  longer 
times,  during  which  all  activities  are  practically  suspended. 
The  majority  of  authentic  cases  of  suspended  activity  indi- 
cate that  beyond  thirty  years  exceedingly  few  seeds  remain 
alive.  It  is  probable  that  during  these  periods  of  appar- 
ently suspended  activity,  some  of  the  vital  processes  still  go 
on,  so  slowly  as  to  be  unnoticeable,  but  resulting  ultimately 
in  the  consumption  or  destruction  of  essential  constituents 
of  actively  living  protoplasm.  Seeds  which  have  lain  dor- 
mant too  long  have,  therefore,  lost  essential  constituents; 
their  actively  living  protoplasm  cannot  be  reconstructed 
when  water  is  added ;  they  have  lost  their  power  of  germi- 
nation. 

The  survival  of  successive  periods  of  drought  by  certain 
species  of  mosses,  liverworts,  lichens,  algae,  and  bacteria  in 
their  vegetative  instead  of  spore  forms  is  even  more  remark- 
able. The  spores  and  seeds  which  survive  periods  of  inac- 
tion have  at  all  times  few  functions  to  perform.  The  vege- 
tative forms  of  mosses,  liverworts,  lichens,  algse,  and  bac- 
teria have  to  perform,  or  to  prepare  for,  all  the  functions 
of  these  organisms.  It  is  all  the  more  remarkable,  there- 
fore, when  they  are  regularly  able  to  survive,  though  all 
their  vital  functions  may  be  suspended.  This  implies  a  con- 
siderably greater  power  of  endurance  than  is  possessed  by 
the  vegetative  parts  of  more  highly  organized  plants.  Yet 
in  California,  and  in  other  parts  of  the  world,  where  there 
are  several  months  in  each  year  when  no  rain  falls,  though 
in  other  months  it  falls  abundantly,  plants  growing  in  un- 
cultivated places  must  be  able  to  adjust  themselves  to  the 
periods  of  enforced  inaction.  So  far  as  our  Pacific  Coast 


INTRODUCTION  11 

plants  are  concerned,  this  is,  however,  an  uninvestigated 
subject,  one  which  offers  many  attractions  to  the  physiolo- 
gist.* 

*  In  the  year  book  of  the  U.  S.  Dept.  of  Agriculture,  1897,  Whitney  re- 
ports (p.  129)  a  comparison  of  the  soils  and  subsoils  of  the  eastern  and 
western  States.  From  this  he  draws  the  conclusion  that  the  absence  of  a 
heavy  subsoil,  and  the  spontaneous  formation  of  a  "mulch"  of  dust  on 
the  surface,  cause  the  plants  of  arid  regions,  and  those  living  where  there 
is  a  long  dry  season,  to  secure  for  their  needs  a  much  larger  percentage  of 
the  total  annual  rainfall  than  is  available  for  eastern  plants.  In  the  East, 
the  heavy  subsoil  drains  off  about  half  of  the  total  rainfall,  and  the  re- 
mainder is  still  further  reduced  by  evaporation.  But  though  all  this  is  un- 
doubtedly true,  yet  the  perennial  plants  themselves  must  also  show  adap- 
tations to  the  climatic  conditions  of  the  far  West  and  of  California,  and 
these  deserve  study  by  physiologists. 


CHAPTER   II 

RESPIRATION 

ENERGY  is  necessary  to  the  operation  of  every  machine, 
whether  it  be  an  engine  or  a  living  organism.  The  engine 
is  supplied  with  energy  directly  or  indirectly  through  the 
intelligent  action  of  a  living  organism  ;  the  living  organism 
supplies  itself  with  energy.  In  both  cases  the  energy  is  ordi- 
narily supplied  by  the  same  means,  mainlyby 


_ 
or  oxidation,  and  the  energy  thus  liberated  is  applied  either 

v~r}ffectly~orindirectly  in  the  same  form.  In  the  case  of  the 
steam-engine  the  energy  or  power  is  applied  indirectly,  the 
heat  resulting  from  the  combustion  of  fuel  being  utilized  in 
the  conversion  of  inelastic  water  into  perfectly  elastic,  com- 
pressed, and  hence  active,  steam.  In  the  living  organism 
the  energy  resulting  from  combustion  is  applied  directly  in 
the  various  kinds  of  work  done  by  the  living  organism 
either  within  or  outside  of  its  own  body. 

Energy  must  be  supplied  to  the  engine  for  two  purposes  : 
first,  to  overcome  the  resistance  (friction,  inertia,  etc.)  of 
its  own  parts;  and  second,  to  enable  it  to  overcome  the 
resistance  offered  by  the  materials  upon  and  in  which  it 
works.  ?£Qr;two^^urpQS£s,  also  energy  must  be  supplied  by 

/the  living  organism  for  its  own  use  :  first,  to  continue  liv- 
ing; and  second,  to  enable  it  to  overcome  the  resistance 

Xoffered  by  the  materials  upon  and  in  which  it  works.  Let 
us  think  first  of  the  need  of  energy  to  continue  living.  Liv- 
ing consists  in  maintaining  the  equilibrium  between  con- 
structive and  destructive  influences.  This  implies  work,  in 
the  physicist's  sense.  So  long  as  the  constructive  influences 
overbalance  the  destructive  the  organism  gains  in  some 
way  —  grows,  or  increases  in  weight,  or  moves,  or  reproduces. 
To  gain  thus,  to  do  any  of  these  things,  the  organism 
needs  energy.  When  the  destructive  influences  overbalance 


RESPIRATION  13 

the  constructive  the  organism  loses  in  some  way— it  ceases 
to  grow,  or  decreases  in  weight,  or  moves  less,  or  fails  to 
reproduce.  The  destructive  influences  result  in  the  libera- 
tion of  energy,  the  constructive  in  the  storing  of  energy/ 
The  liberation  of  energy  implies  previous  construction,  for 
the  generation  of  energy  from  nothing  is  inconceivable. 
The  chief  source  of  energy  in  the  organism  is  combustion, 
the  destruction  by  oxidation  of  not  merely  already  exist- 
ing, but  really  of  previously  formed  substances.  The  direct 
result  of  oxidation,  is  the  evolution  of  heat.  \^ 
-  To  the  oxidation  which  goes  on  within  the  living  organ- 
ism is  given  the  name  RESPIRATION.  The  interchange  of 
gases  between  the  blood  of  higher  animals  and  the  air, 
which  takes  place  at  the  lungs  or  gills,  is  but  a  part  of  the 
process  of  respiration.  The  oxygen  taken  up  by  the  blooc 
at  those  surfaces  exposed  to  the  air  is  transferred  by  the 
circulation  of  the  blood  to  the  tissues  and  cells  by  which  it 
is  used.  The  oxygen  is  used  by  combining  it  with  other 
substances/ by  oxidizing  these  substances.  Respiration  is 
physiological  oxidation,  or  combustion,  as  distinguishec 
from  such  oxidations  as  take  place  under  ordinary  condi- 
tions spontaneously.  The  element  sodium,  when  exposed  to 
the  air,  will  unite  with  the  oxygen  without  heat,  light,  or 
any  other  known  form  of  energy  being  applied  to  encourage 
the  union.  Animal  and  vegetable  substances  will  not  so 
unite,  the  oxidation  must  be  encouraged,  and  it  will  take 
place  outside  the  living  body  only  at  a  temperature  con- 
siderably higher  than  that  developed  within  the  body  of  any 
living  organism.  In  respiration  we  have,  then,  a  process 
which  differs  from  ordinary  oxidation  in  the  conditions 
under  which  it  takes  place.  The  results,  however,  are  the 
same.  ^In  respiration  we  have  to  do  not  only  with  the 
affinity  of  oxygen  for  certain  elements  and  compounds,  but 
with  the  need  of  the  living  organism_fQiL_ene£gy.  The  or- 
ganism must  be  actively  living  in  order  that  the  oxygen  of 
the  air  may  be  made  to  unite  with  those  substances  in  the 
body  from  which  energy  is  to  be  liberated.  Respiration  may 
be  artificially  continued,  with  the  liberation  of  energy,  only 
for  a  short  time  after  the  death  of  the  organism  as  a 


14  PLANT  PHYSIOLOGY 

whole,  only  until  the  death  of  the  cells  composing  the  body. 
The  cells  need  free  oxygen  during  life;  they  remove  it  by 
combining  it  with  complex  combustible  compounds.  So 
long  as  the  removal  of  free  oxygen  by  combining  it  with 
other  substances  takes  place,  so  long  will  oxygen  continue, 
in  obedience  to  the  laws  governing  the  diffusion  of  gases,  to 
enter  the  cells.  The  combination  of  oxygen  with  combusti- 
ble substances  at  temperatures  below  those  at  which  these 
substances  would  spontaneously  oxidize,  takes  place  in  liv- 
ing cells  and  is  accomplished  only  by  them. 

Though  the  living  cell  is  supplied  with  oxygen  by  purely 
physical  means,  by  diffusion,  the  continued  supply  of  new 
molecules  of  oxygen  is  contingent  upon  its  continued  con- 
sumption by  the  cell.  The  supply  must  make  good  the  lack 
produced  by  oxidation,  the  rate  of  oxidation  must  equal  the 
demand  for  energy,  and  the  amount  of  work  done  in  the 
cell  will  always  be  directly  proportioned  to  the  amount  of 
energy  it  can  liberate.  The  initiative,  however,  will  always 
come  from  the  living  cell,  because  oxidation  by  respiration 
is  not  dependent  solely  upon  the  mutual  affinities  of  oxygen 
and  the  substances  to  be  oxidized.  The  activity  of  the  cell 
controls  the  activity  of  respiration  ( not  vice  versa ) ,  and  the 
supply  of  oxygen,  normally  exactly  equal  to  the  consump- 
tion, is  also  controlled  by  the  cell.  An  excessive  supply  of 
oxygen,  which  for  experimental  purposes  may  be  artificially 
furnished,  will  affect  the  respiratory  only  as  it,  at  the  same 
time  affects  the  other  activities  of  the  cell.  Each  kind  of 
cell  and  each  kind  of  organism  will  have  its  own  optimum, 
maximum,  and  minimum,  any  departures  from  which  will 
characteristically  affect  the  cell  and  the  organism.  The 
percentage  of  oxygen  in  atmospheric  air  (about  20%)  is 
approximately  the  optimum  for  the  majority  of  land  organ- 
isms, though  for  a  small  number  this  amount,  and  even 
much  less  than  this,  is  fatal.  The  amount  of  available  free 
oxygen  necessarily  varies  with  the  habitat,  plants  living  on 
mountain-tops,  and  in  water,  having  less  than  those  living  on 
the  land  at  ordinary  elevations.  But  the  successful  existence 
of  plants  at  different  elevations  and  depths  shows  that  they 
are  capable  of  supplying  themselves  with  what  they  need. 


RESPIRATION  15 

Within  the  limits  in  which  normal  respiration  is  possible, 
plants,  and  hence  their  component  cells,  will  supply  them- 
selves with  adequate  amounts  of  oxygen,  an  excessive  pro- 
portion suddenly  supplied  causing,  in  some  plants,  merely  a 
greater  accumulation  of  oxygen,  unaccompanied  by  greater 
respiration,  *  in  the  cells ;  in  others,  more  rapid  respiration 
as  indicated  by  higher  temperature,  etc.  On  the  other  hand, 
sudden  reduction  of  the  amount  of  oxygen  may  cause  a 
diminution  of  the  respiratory  activity. 

So  long  as  the  general  conditions  for  life  continue  in  ade- 
quate degree,  there  is  no  cessation  of  respiration  in  plants 
or  in  animals.  This  is  true  of  the  resting  forms — buds, 
bulbs,  tubers — although  in  them  the  rate  of  respiration  is 
much  lower  than  in  active  forms.  It  is  possible  to  force 
these  resting  forms  by  various  means  into  activity,  but  it 
is  not  possible,  after  the}'  mature,  to  continue  the  rate  of 
respiration  which  they  possessed  during  their  development. 
The  forming  bud,  bulb,  or  tuber  is  composed  of  cells  ac- 
tively working,  needing  much  energy  with  which  to  work, 
and  respiring  rapidly  in  order  to  secure  this.  With  the  ap- 
proach of  maturity  of  the  parts,  the  work  to  be  done,  the 
need  of  energy,  and  the  rate  of  respiration,  diminish,  no 
matter  how  favorable  to  continued  activity  the  external 
conditions  may  be.  This  decrease  in  respiration,  and  in  the 
other  functions,  is  largely  due  to  the  influences  of  its  envi- 
ronment upon  all  the  functions  of  the  organism  as  a  whole. 
The  organism  prepares  itself  for  the  regularly  recurring 
periods  of  drought,  heat,  or  cold ;  one  activity  after  another 
is  suspended  accordingly.  After  a  period  of  rest,  it  is  possi- 
ble to  force  these  forms  into  activity  even  after  the  lapse  of 
much  less  than  the  usual  time.  Lilac  branches,  cut  from  the 
bushes  late  in  autumn,  can  be  forced  by  Christmas  time  to 
develop  the  flowers  and  leaves  already  formed.  This  forcing 
is  accomplished  by  placing  the  ends  of  the  cut  branches  in 
jars  of  water  and  keeping  them  in  a  warm,  damp,  not  too 
brightly  lighted  place.  This  experiment,  familiar  enough  to 
the  florist,  results  in  the  resumption  of  active  respiration 

*  Pfeffer?8  Handbuch  der  Pflanzenphysiologie,  2te  Auflage,  Bd.  I.,  p. 
547,  1897.  Eng.  tranel.  by  Ewart,  vol.  I.,  p.  539,  1900. 


16  PLANT  PHYSIOLOGY 

after  a  period  of  tardy  respiration  shorter  than  usual;  it 
does  not  continue  the  respiratory  and  other  activities  at  the 
rate  prevailing  during  the  formation  of  the  parts. 

Respiration  is  a  process  conducted  and  regulated  by  the 
living  protoplasm  of  the  cells.  It  will  not  go  on  indefinitely 
and  independently  however  favorable  the  physical  conditions 
may  be.  It  can  be  artificially  increased  only  by  stimulat- 
ing the  protoplasm  to  greater  activity ;  it  will  be  increased 
whenever  the  protoplasm  becomes  more  active.  The  respira- 
tion of  plants  and  plant-cells  can  be  artificially  reduced  only 
by  reducing  the  general  activity  of  the  living  protoplasm. 
This  may  be  accomplished  by  the  same  means  as  the  animal 
physiologist  employs — by  applying  a  local  or  general  anaes- 
thetic, by  lowering  the  temperature,  by  preventing  move- 
ment, etc.  In  most  plants  and  plant  parts  the  forced  cessa- 
tion of  normal  respiration  immediately  precedes,  and  is 
itself  the  cause  of,  death.  In  certain  plants — for  example, 
germinating  peas — intramolecular  respiration*  may  tem- 
porarily take  its  place;  in  certain  others — for  example, 
anaerobic  bacteriat— intramolecular  respiration  is  the  nor- 
mal mode. 

Normal  respiration  cannot,  however,  be  entirely  suspended 
in  plants  or  plant  parts  without  profound  changes  taking 
place  which  sooner  or  later  will  result  in  death.  The  reduc- 
tion of  normal  respiration  to  an  extremely  low  rate,  if  not 
its  entire  suspension,  unaccompanied  by  any  other  means  of 
obtaining  energy,  regularly  takes  place  in  the  ripe  seed,  but 
only  when  the  cells  composing  the  seed  lose  the  water  which 
is  an  essential  constructive  constituent  of  living  proto- 
plasm. 

In  warm-blooded  animals  the  object  of  respiration  is  two- 
fold— the  maintenance  of  a  certain  (normal)  body-tempera- 
ture and  the  production  of  energy  for  doing  work.  In  cold- 
blooded animals  and  in  plants  the  object  of  respiration  is 
solely  the  latter.  ;The  average  body-temperature  of  plants 
is  in  general  nearly  the  mean  daily  temperature  of  their 
environment,  and  it  will  vary  within  certain  limits  accord- 
ingly. The  variation  in  the  body-temperature  of  plants  will 
*  See  page  27.  t  See  page  26. 


RESPIRATION  17 

be  large  or  small  according  to  the  environment.  Submerged 
aquatics  will  vary  least,  floating  aquatics  more,  and  ter- 
restrial plants  most ;  but  as  the  temperature  of  small,  still 
bodies  of  water  (pools,  etc.)  varies  considerably,  so  the 
body-temperature  of  the  organisms  living  therein  will  vary, 
warmed  by  the  sun  and  cooled  during  the  night.  The  body- 
temperature  of  the  larger  terrestrial  plants  is  likely  to  be 
higher  at  night  (except  hi  the  exposed  surfaces)  and  lower 
in  the  day,  than  that  of  the  surrounding  air.  Owing  to  the 
very  great  external  surface  of  the  larger  plants  in  propor- 
tion to  their  mass,  radiation  from  them  is  rapid,  and  a 
body-temperature  independent  of  their  environment  could  be 
maintained  only  at  great  expense  of  material  laboriously 
collected  and  elaborated.  Plants  work  economically,  must 
do  so,  and  such  extravagance  is  avoided. 

Heat  is  the  form  in  which  the  energy  set  free  by  respira- 
tion usually  makes  itself  evident,  but  it  does  not  necessarily 
follow  that  only  so  much  energy  is  liberated  as  is  recogniz- 
able as  heat,  or  that  this  is  the  only  form  in  which  energy 
is  liberated.  Only  that  energy  becomes  evident  as  such 
which  is  not  at  once  used.  To  determine  the  amount  of 
energy  liberated  in  respiration,  it  is  necessary  to  know  and 
to  measure  the  material  products  of  respiration. 

The  substances  ordinarily  engaged  in  the  process  of  physi- 
ological oxidation  are  the  highly  complex  nitrogenous  and 
non-nitrogenous  compounds  elaborated  by  the  organism. 
The  ordinary  products  are  carbon-dioxide,  water,  and 
various  small  amounts  of  several  other  substances,  e.  g. 
oxalic  acid.  Since  the  production  of  energy  rather  than  of 
any  particular  compounds  is  what  is  striven  for  in  respira- 
tion, and  since  the  substances  acted  upon  by  free  oxygen 
are  different  in  different  plants  and  cells,  the  products  will 
differ  accordingly. 

Although  the  oxidation  of  nitrogenous  matters  also  takes 
place,  it  is  mainly  the  non-nitrogenous  contents  of  the  living 
cell  which  are  involved  in  physiological  oxidation.  In  the 
animal  body,  the  oxidation  of  organic  nitrogenous  com- 
pounds ( proteids )  results  in  the  production  of  urea  and  of 
other  similar  substances  no  longer  usable  and  presently 
2 


18  PLANT  PHYSIOLOGY 

cast  off  from  the  body.  In  plants,  the  elimination  of 
these  products  is  more  economically  accomplished,  for  they 
furnish  the  foundations  for  the  re-synthesis  of  albumi- 
nous compounds,  as  will  be  discussed  under  the  subject 
of  nutrition  (see  page  71).  These  waste  substances  are 
removed  by  transforming  them  synthetically  into  useful 
compounds. 

The  non-nitrogenous  substances  which  become  oxidized 
are  the  fats  and  oils,  the  starches  and  sugars.  The  oxida- 
tion may  first  convert  the  hydrocarbons  into  carbo-hy- 
drates, with  the  liberation  of  energy  and  the  formation  of 
by-products,  carbo-hydrates  and  by-products  then  becoming 
still  further  oxidized  with  the  liberation  of  still  more  energy. 
While  respiration  is  going  on,  the  other  functions  in  opera- 
tion also  may  involve  the  use,  by  chemical  change,  of  some 
of  each  substance  produced  in  respiration  and  the  formation 
in  the  cell  of  other  substances  not  the  products  of.  respira- 
tion at  all.  It  is  therefore  evident  that  to  ascertain  the 
material  products  of  respiration  is  hardly  less  difficult  than 
to  determine  the  amount  of  energy  liberated.  To  isolate 
any  physiological  process  for  purposes  of  study  is  impossi- 
ble, for  each  process  is  normal  only  when  accompanied  by 
all  the  processes  normally  going  on  at  the  same  time.  The 
products  of  one  set  of  chemical  activities  in  the  living  body 
may  enter  wholly  or  partially,  simultaneously  or  succes- 
sively, into  other  chemical  activities.  The  end  products  can 
be  recognized  and  measured  with  comparative  ease,  but  to 
tell  exactly  where  or  how  they  are  formed  is  much  more 
difficult  and  not  now  entirely  possible. 

Water  and  carbon-dioxide  gas  are  the  chief  products  of  the 
physiological  as  also  of  other  forms  of  combustion  of  car- 
bon-containing bodies.  They  are  formed  whenever  a  suffi- 
cient amount  of  oxygen  is  united  with  the  higher  carbon 
compounds.  In  organisms  living  under  such  conditions  that 
the  air  can  penetrate  to  all  their  parts,  enough  oxygen  will 
always  be  present  for  such  complete  decomposition.  Under 
ordinary  conditions  oxygen  does  not  unite  of  itself  with  the 
combustible  compound,  and  if  active  (nascent)  oxygen  is 
present  at  all  in  the  cell  it  is  only  in  amounts  insufficient  to 


RESPIRATION  19 

accomplish  the  whole  result.  *  The  union  of  ox ygen  and  the 
substances  to  be  oxidized  is  accomplished  bythe  living  cell. 
HowThts  is  done  is  not  known,  though  conjectures  are  not 
lacking.  Whether  the  combustible  substances  and  the  oxy- 
gen are  divided  into  such  small  particles  and  so  intimately 
associated  in  the  living  protoplasm  that  union  takes  place 
spontaneously,  or  whether  the  oxidation  is  accomplished  by 
enzyms,  or  whether  more  readily  or  spontaneously  oxidiza- 
ble  substances  are  first  formed  from  sugar  ( perhaps  by  the 
action  of  an  enzym),  is  not  known,  though  there  is  a  cer- 
tain amount  of  evidence  in  favor  of  each  hypothesis.  All 
that  is  known  is  that  sugar,  or  some  similar  substance,  and 
oxygen,  unite,  forming  as  end-products  mainly  carbon- 
dioxide  and  water.  The  following  reaction,  without  indicat- 
ing what  intermediate  stages  there  may  be,  if  there  are  any, 
shows  the  material  results : 

C.  H,s  O.  +  6  Ot  ( +  Aq )  =  6  CO,  +  6  H,  O  ( +Aq ) 

(\([.  ivjnvsHiits  the  water  in  which  the  su<i'ur  is  dissolved 
in  the  cell :  it  does  not  enter  the  reaction.  The  6  H2  O  may 
unite  with  this  or  these  molecules  may  pass  off  as  vapor,  as 
in  ordinary  combustion,  being  kept  in  a  state  of  vapor  by 
the  heat  liberated.) 

Since  other  substances  than  sugar  are  also  oxidized  physi- 
ologically in  the  cell,  there  are  also  other  products  of  sorts 
and  in  quantities  varying  with  these.  The  commoner  of 
these  minor  products  are  oxalic,  malic,  and  citric  acids, 
which  accumulate  in  considerable  quantities  in  certain 
plants  (e.  g.  in  the  leaves  of  Oxalis  acetocella  and  in  the 
Crassulacese,  in  apples,  etc.,  and  in  the  citrous  fruits — 
lemons,  limes,  oranges,  etc. ) ,  or  are  converted  into  salts 
( e.  g.  calcic  oxalate,  crystallizing  out  of  the  solutions  in 
which  it  is  formed  in  the  cell),  or  undergo  other  changes 
( e.  g.  further  oxidation ) . 

In  all  organisms  the  oxidation  of  nitrogenous  as  well  as 
non-nitrogenous  compounds  occurs  in  normal  respiration. 
The  proportional  amounts  of  these  two  groups  of  com- 
pounds physiologically  oxidized  varies  with  different  organ- 

*  Pfeffer,W.    Oxydationsvorgange.    Physiol.  of  Plants,  Vol.  I.,  pp.  54-56. 


20  PLANT  PHYSIOLOGY 

isms.  In  the  majority  only  organic  and  highly  complex 
compounds  are  made  to  yield  the  needed  energy,  but  in 
some  organisms  much  simpler  inorganic  compounds  suf- 
fice, and  in  a  few  now  known  (more  may  later  be  dis- 
covered) the  carbon-containing  compounds  are  not  used 
at  all. 

The  nitro-bacteria,  as  shown  first  by  Winogradsky,  *  oxi- 
dize simpler  nitrogen  compounds  in  order  to  liberate  the 
energy  which  they  need,  employing  carbon-compounds  only 
in  the  synthesis  of  food  to  be  used  in  the  construction  of 
their  living  bodies.  One  set  of  nitro-bacteria  oxidize  am- 
monia, or  compounds  of  ammonia,  to  nitrous  acid,  the  first 
and  last  steps  of  the  process  being  indicated  by  the  reac- 
tion— 

2  NH4  OH  +  3  O2  =  2  HNO.  +  4  H2  0 

Another  set  oxidize  the  nitrous  acid,  or  its  salts,  to 
nitric  acid  thus — 

2  HN02  -f  02  =  2  HN03 

The  sulphur-bacteria  ( Beggiatoa,  etc. )  obtain  most  if  not 
all  of  their  kinetic  energy  by  oxidizing  sulphur  compounds. 
Thus  they  precipitate  sulphur  in  their  own  bodies  by  oxidiz- 
ing the  sulphuretted  hydrogen  ( H.,  S )  present  in  the  waters 
in  which  they  live.f  If  the  supply  of  the  gas  remain  suffi- 
cient, the  sulphur  will  accumulate  as  a  reserve  supply  in  the 
cells ;  if  it  decrease,  the  reserve  sulphur  will  be  oxidized  and, 
uniting  with  water,  will  form  sulphuric  acid,  or  its  salts, 
thus — 

S  +  02  (  +  Aq)=S02  (+Aq) 
2S02  +  02  (+Aq)=2S03  (  +  Aq) 
S03  +  H2  0  (+  Aq)  =  H2  S04  (  + Aq) 

Those  bacteria  (e.g.  Crenothrix)  which,  living  in  water 
rich  in  iron,  deposit  iron  in  some  form  in  or  upon  their  own 
bodies,  may  obtain  their  kinetic  energy  by  physiologically 

*  Winogradsky,  S.  Recherches  sur  les  organismes  de  la  nitrification.  An- 
nales  de  1'Inst.  Pasteur,  IV.,  V.,  1889-91— and  other  papers. 

t  Winogradsky,  S.  Uber  Schwefelbakterien.  Botanische  Zeitung,  1887. 
Beitrage  zur  Morphologic  u.  Physiologic  der  Bakterien.  Leipzig,  1888. 


RESPIRATION  21 

oxidizing  ferrous  compounds,  presumably  ferrous  oxide  to 
ferric  oxide.* 

Other  bacteria  may  be  discovered  which,  needing  carbon 
and  nitrogen  compounds  only  to  supply  the  constructive 
elements  of  protoplasm,  obtain  energy  by  oxidizing  other 
substances  present  in  solution  in  the  waters  in  which  they 
live,  and  therefrom  absorbed  into  their  own  bodies. 

The^essential  product  of  respiration,  the  one  which  dis- 
tinguishes ilTTrom  all  the  other  functions  of  the  living  or- 
ganism, jsjdyaetie  energy .  The  material  products  vary  in 
kind  and  in  quantity  according  to  the  nature  of  the  organ- 
ism and  the  substances  which  can  be  affected.  Ordinarily 
these  substances  are  complex  compounds  elaborated  within 
the  body  of  the  respiring  plant.  That  this  is  not  essen- 
tial is  shown  by  the  successful  existence  of  bacteria  which 
oxidize  nitrogen,  sulphur,  iron,  and  possibly  compounds  of 
other  elements.  These,  though  necessarily  absorbed  before 
they  can  be  acted  upon,  are  not  first  elaborated  by  the  cell. 

Free  oxygen  is  not  necessary  to  all  organisms  or  to  all 
cells.  The  haemoglobin  of  the  blood  is  a  complex  com- 
pound from  which  some  of  the  oxygen,  only  loosely  held, 
can  be  readily  given  off  where  oxidation  for  the  supply  of 
energy  is  needed.  Similarly  the  color  products  of  certain 
bacteria  (e.g.,  Bacillus  brunneus)  are  reserves  of  oxygen 
which  become  used  when  there  is  no  longer  an  adequate 
supply  of  free  oxygen. f  From  colorless  compounds  also, 
the  cells  at  depths  in  the  tissues  of  animals  (perhaps  also 
of  plants^),  to  which  free  oxygen  penetrates  only  in  in- 
sufficient amounts  if  at  all,  obtain  by  decomposition  the 
energy  needed.  These  decompositions  are  not  necessarily 

*  Winogradsky,  S.  Uber  Eisenbakterien.  Bot.  Zeitung,  1888.  Molisch, 
H.  Die  Pflanze  in  ihren  Beziehungen  zum  Eisen,  Jena,  1892.  See  also 
Miyoski,  M.  Studien  iiber  die  Schwefelrasenbildung  und  die  Schwefel- 
bacterien  der  Thermen  von  Yumoto  bei  Nikko  and  Uber  das  massenhafte 
Vorkommen  von  Eisenbacterien  in  den  Thermen  von  Ikao.  Journal  Coll. 
Science,  Imp.  Univ..  Tokyo,  vol.  X.,  pt.  II.,  1897. 

\  Ewart,  A.  J.  On  the  evolution  of  oxygen  from  colored  bacteria.  Jour- 
nal  of  the  Linnean  Society,  XXXIII.,  p.  123,  1897. 

t  Pfeffer,  W.    Berichte  der  Math-phys.  Classe  der  Konigl.  Sachs. 
der  Wissensch.  zu  Leipzig,  27  Juli,  1896,  p.  383. 


22  PLANT  PHYSIOLOGY 

effected  to  secure  oxygen  for  oxidation  of  other  substances, 
but  the  decompositions  themselves  release  as  kinetic  the 
potential  energy  which  was  needed  to  hold  the  complex 
substances  together. 

The  mutual  attraction  of  one  atom  of  carbon  and  two  of 
oxygen  is  so  great  that  the  molecule  of  carbon-dioxide 
is  very  stable  as  well  as  very  simple,  for  the  "affinities" 
of  the  carbon  and  oxygen  are  "satisfied."  In  such  complex 
compounds  of  carbon,  hydrogen,  and  oxygen  as  the 
starches,  sugars,  etc.,  the  affinities  of  the  component  ele- 
ments are  not  satisfied ;  the  compounds  are  much  less  stable, 
as  is  shown  by  their  ability  to  take  up  more  oxygen.  At 
ordinary  temperatures,  however,  and  under  ordinary  condi- 
tions, these  compounds  are  stable.  They  owe  their  stability 
to  the  mutual  affinities  of  their  component  atoms,  which 
exert  an  attraction  upon  one  another  sufficiently  powerful 
to  hold  them  together  in  definite  form.  When  the  atoms 
are  separated  from  one  another  and  arrange  themselves 
closer  together  in  simpler  forms  in  space,  their  "bonds"  or 
"affinities"  are  more  completely  "satisfied,"  they  unite  more 
perfectly,  oxidation  takes  place  in  the  rearrangement,  and 
energy  is  accordingly  liberated  and  made  available  for  other 
purposes.  Energy  is  "stored"  in  the  starch,  or  sugar,  or 
oil  molecule;  the  kinetic  energy  (solar  or  other)  employed 
in  the  construction  of  the  molecule  remains  in  it  as  poten- 
tial energy,  holding  the  atoms  together.  The  destruction  of 
the  molecule  results  in  the  liberation  of  so  much  kinetid 
energy  as  was  employed  in  constructing  it  from  the  simple 
compounds  worked  upon. 

The  complete  oxidation  or  combustion  of  a  gram  of 
dextrose  (sugar),  resulting  in  the  formation  of  carbon- 
dioxide  and  water  as  represented  in  the  following  reaction— 

C6  H,,  Ofl  +  6  Oa  =  6  C02  +  6  H,  O 

liberates  3,939  small  or  ordinary  calories,  or  mechanical 
units  of  energy  in  the  form  of  heat.  *  ( A  calorie  ( c )  is  the 

*  Rechenberg,  C.  von.  Uber  die  Verbrennungswarme  organischer  Ver- 
bindungen.  Inaug.  Diss.,  Leipzig,  1880.  See  also  Pembry,  M.  on 
Animal  Heat  in  Schafer's  Text-book  of  Physiology,  vol.  I.,  Edinburg, 
London,  New  York,  1898.  Here  the  literature  is  fully  given. 


RESPIRATION  23 

heat  required  to  raise  1  gram  of  water  1°  C.  in  temperature, 
a  great  calorie  (C)  is  the  heat  required  to  raise  1,000  gr. 
(1  Kilo)  of  water  1°  C.*).  But  since  a  gram  of  dextrose 
may  contain  a  very  different  number  of  molecules  from  a 
gram  of  any  other  substance,  and  since  the  liberation  of 
energy  is  due  to  the  oxidation  of  molecules  rather  than  of 
mere  weights  of  a  substance,  it  is  well  to  convert  these 
figures  into  a  form  which  will  enable  one  to  compare  the 
yield  in  energy  when  the  substances  are  similar  in  quanti- 
ties. Thus,  by  using  the  molecular  weights  of  the  com- 
pounds to  indicate  the  number  of  grams,  we  have  a  com- 
mon basis  for  the  comparison  of  the  heats  liberated,  namely, 
the  gram-molecule.  Thus  the  molecular  weight  of  dextrose 
is  180,  obtained  thus- 
atomic  weight  of  C  =  12  C6  =  72 
"  "  H  =  1  H12  =  12 
"  "  O  =  16  0.  =  96 


180  =  molecular 
weight  of  dextrose  (C6  H12  06) 

3,939  calories  x  180  =  709,020  calories  =  709. 02  Calories. 
The  heat  of  combustion  (complete  oxidation)  of  1  gram- 
molecule,  i.  P.  of  180  grams  of  dextrose,  is  then  709.00 
Great  Calories  (C.) 

This  reaction  and  the  production  of  this  amount  of  heat 
take  place  only  in  the  presence  of  sufficient  quantities  of  free 
oxygen.  Molecules  more  complex  than  those  of  carbon- 
dioxide  and  water,  though  simpler  than  sugar,  may  be 
formed  from  sugar  without  free  oxygen  or  with  free  oxygen 
in  smaller  proportions  than  6  :1.  Complete  oxidation  (nor- 
mal respiration)  yields  the  largest  amount  of  energy,  less 
profound  changes  yield  less  energy.  Thus  the  decomposi- 
tion of  sugar  by  yeasts,  according  to  the  following  reaction, 

*  It  may  be  interesting  to  compare  the  amounts  of  heat  liberated  by 
burning  1  gram  of  different  substances,  thus : 
1  gram  bread  crumbs  (contain  some  proteid)  3984  calories. 
1     "       butter  7264       " 

1     "       wood-charcoal  (mainly  carbon)  8080       " 

1     "       hydrogen  33881       " 


24  PLANT  PHYSIOLOGY 

which  represents  only  in  simplest  terms  the  nature  of  the 
chemical  changes — 

C.  H,,  06  =  2  C,  H6  0  +  2  CO, 

(alcohol) 

forming  without  oxygen  two  molecules  of  alcohol  and  two 
of  carbon-dioxide  from  one  molecule  of  dextrose — yields  only 
67  calories  per  gram-molecule.* 

The  decomposition  of  one  molecule  of  dextrose  into  one 
molecule  of  butyric  acid,  two  of  carbon-dioxide,  two  of 
hydrogen,  which  is  accomplished  by  a  considerable  number 
of  bacteria  and  may  be  represented  by  the  following  reac- 
tion— 

C6  H12  06  =  C4  He  02  +  2  C02  +  2  H2 
(butyric  acid) 

yields  about  75  calories  per  gram-molecule,  f 

Bacteria  forming  acetic  acid,  acting  on  dilute  solutions  of 
ethyl  alcohol  in  the  presence  of  free  oxygen,  partially  oxidize 
the  alcohol  and  decompose  it  into  acetic  acid  and  water, 
thus— 

C,  H.O.+  0,  =  C,  H4  0,  +  H.O 

liberating  125  calories^ ;  but  if  the  alcohol  were  completely 
oxidized,  as  in  ordinary  combustion,  the  reaction  would  be 

C2  H6  0  +  3  02  =  2  C02  +  3  Ha  O 

and  the  heat  liberated  would  be  nearly  three  times  as  much, 
about  325  calories  per  gram-molecule. 

In  these  figures  we  have  indices  of  the  relative  values  of 
complete  and  incomplete  oxidations,  and  of  oxidations  and 
decompositions,  as  sources  of  energy  in  the  form  of  heat. 
These  figures  are  indices,  to  be  trusted  only  so  far  as  rela- 
tive, not  exactly  proportional,  values  are  concerned ;  for  the 
chemist  can  control  all  the  conditions  under  which  he  makes 
a  combustion  in  his  laboratory  and  determines  the  number 
of  heat-units  liberated ;  he  can  so  regulate  the  process  that 
there  shall  be  no  by-products  and  that  no  other  compounds 
are  included  in  the  reaction  than  those  upon  which  he  has 

*  Rechenberg,  1.  c.,  p.  66.  t  Ibid.,  p.  67. 

\  Quoted  from  Berthelot  in  Biedermann's  Chemiker-Kalender  for  1897, 
p.  193  of  the  Beitrage. 


RESPIRATION  25 

determined  to  experiment.  In  the  plant,  on  the  contrary, . 
other  substances  than  dextrose  may  become  oxidized,  or  the 
oxidation  of  dextrose  may  be  incomplete.  In  the  laboratory 
one  can  deal  with  definite  quantities  of  isolated  substances ; 
in  the  living  organism  indefinitely  known  quantities  of 
many  substances  together  are  acted  upon.  Animal  physi- 
ologists have  done  much  more  in  this  direction  than  have 
plant  physiologists,  and  the  high  organisms  which  they 
study  are  better  suited  for  the  purpose  than  are  plants. 
The  relatively  high  body-temperatures  of  warm-blooded  ani- 
mals permit  direct  temperature  determinations  from  weighed 
quantities  of  known  foods  eaten,  as  well  as  calculations 
from  the  amounts  of  oxygen  needed  to  effect  combustions 
or  decompositions.  Thus  the  animal  physiologist  can  check 
the  results  obtained  by  one  method  with  those  obtained  by 
other  methods.  The  results  of  animal  physiologists  indicate 
that  only  about  95%  of  the  calculated  yield  of  energy  from 
oxidation*  appears  as  heat.  So  then  we  must  regard  these 
figures  as  somewhat  too  high,  but  their  suggestive  value  is 
great  whatever  must  be  admitted  as  to  their  exact  numeri- 
cal value. 

The  larger  organisms  demand  for  the  normal  execution  of 
their  functions  more  energy  than  can  be  supplied  by  the  re- 
arrangement of  the  component  atoms  of  already  furnished 
molecules ;  they  must  oxidize  these  molecules,  and  the  more 
complete  the  oxidation,  the  greater  the  amount  of  energy 
liberated.  Some  of  the  smaller  organisms  supply  them- 
selves with  adequate  amounts  of  energy  by  the  destruction 
of  complex  compounds  within  their  own  living  cells.  Proba- 
bly some  of  the  cells  of  all  organisms  have  recourse,  at 
times  at  least,  to  the  same  means  of  securing  needed  energy, 
and  when  free  oxygen  is  not  obtainable  the  majority  of 
organisms  can  continue  living  for  a  time  by  so  doing. 
From  this  the  general  inference  may  safely  be  drawn,  that 
the  ability  to  obtain  needed  energy  by  the  destruction  of 
complex  substances  in  the  cells  is  inherent  in  all  organisms ; 
that  in  the  majority  of  organisms  and  of  their  component 

*  See  Pembry  in  Schafer's  Physiology,  vol.  I.,  pp.  836-7. 


26  PLANT  PHYSIOLOGY 

cells  this  power  is  little  needed  and  hence  is  practically  un 
developed ;  but  that,  owing  to  the  position  of  some  cells 
deep  in  the  tissues  of  many  of  the  larger  organisms,  and  tc 
the  peculiar  habits  of  some  of  the  lowest  organisms,  thes< 
are  obliged  to  obtain  needed  energy  in  this  way  and  hav< 
developed  their  inherent  powers  to  a  high  degree. 

INTRAMOLECULAR  RESPIRATION  is  the  name  given  to  this 
mode  of  respiration,  a  term  not  entirely  satisfactory,  for  it  it 
not  explicitly  descriptive.  The  German  term  SpaltungsatL 
mung  is  in  this  regard  much  more  satisfactory,  but  it  is  nol 
concisely  translatable.  Ordinary  respiration,  physiologica 
oxidation  or  physiological  combustion,  is  aerobic  respira 
tion — respiration  which  is  dependent  upon  free  oxygen,  anc 
which  yields  the  needed  kinetic  energy  only  by  the  union  o 
free  oxygen  with  combustible  substances.  Intramoleculai 
respiration,  physiological  simplification  of  complex  sub 
stances,  physiological  rearrangement  of  atoms,  is  anaerobic 
respiration — respiration  which  takes  place  only  when  fre< 
oxygen  is  present  in  insufficient  quantities  or  is  altogethei 
absent.  The  results  of  the  two  processes  are  the  same  ir 
kind— the  liberation  of  the  kinetic  energy  necessary  to  con 
tinue  living— but  not  the  same  in  degree,  as  the  figures 
above  quoted  show. 

It  is  now  about  one  hundred  years  since  intramoleculai 
respiration  was  first  observed,*  but  only  within  the  lasl 
few  years  has  the  connection  between  the  two  means  o 
securing  energy  been  demonstrated.  In  addition  to  the  ani 
mal  and  plant  physiologists,  our  present  knowledge  is  du< 
also  to  Pasteur  and  other  bacteriologists,  for  they  hav< 
shown  the  peculiar  habit  of  a  large  number  of  micro-organ 
isms  in  actively  living  only  where  free  oxygen  is  absent.  W< 
have  a  chain  of  allied  processes :  first,  physiological  oxida 
tion,  or  what  may  be  called  inter-molecular  respiration,  th( 
normal  respiration  of  most  organisms;  second,  physiologi- 
cal rearrangement  of  atoms  into  simpler  molecules,  intra 
molecular  respiration,  the  mode  of  respiration  which  man} 
cells  and  a  good  many  organisms  have  recourse  to  undei 


*  Hollo.    Annales  de  Chimie,  t.  25,  1798. 


RESPIRATION  27 

stress  of  circumstances ;  third,  physiological  rearrangement 
of  atoms  into  simpler  molecules,  also  really  intramolecular 
respiration,  the  anaerobic  normal  respiration  of  a  compara- 
tively small  number  of  invariably  low  organisms. 

INTRAMOLECULAR   RESPIRATION 

Having  examined  the  first  of  these  allied  processes  as 
thoroughly  as  our  limits  allow,  let  us  pass  to  the  second. 
From  experiments  hitherto  conducted,  it  would  seem  that 
the  germinating  seeds  of  higher  plants  are  better  able  to 
survive  without  a  copious  supply  of  oxygen  than  are  the 
other  parts.  This  is  what  might  be  expected,  for  the  em- 
bryo in  the  seed,  when  it  becomes  active  in  germination,  is 
a  very  vigorous  organism,  usually  well  supplied  with  just 
such  foods  as  may  be  readily  broken  down  into  simpler 
compounds.  The  seeds  of  pea,  for  example,  stimulated  to 
germinate  by  being  soaked  in  water  at  room-temperature 
for  twelve  or  fifteen  hours,  will  continue  to  respire  actively 
for  forty-eight  hours  or  longer,  even  in  a  vacuum,  producing 
carbon-dioxide  in  nearly  the  same  quantity  as  under  the 
same  conditions  of  temperature,  etc.,  in  ordinary  air.  Of 
course  some  air  containing  free  oxygen  will  be  carried  into 
the  vacuum  by  the  peas,  but  this  will  very  soon  be  entirely 
consumed  in  normal  respiration,  and  the  continued  supply 
of  energy  must  be  obtained  by  intramolecular  respiration.  * 
Comparative  investigations  have  shown  that  different 
plants  and  different  organs  vary  considerably  in  their  abil- 
ity under  stress  to  substitute  intramolecular  for  normal 
respiration,  and  that  in  very  few  of  the  higher  plants  is 
intramolecular  respiration,  as  measured  by  the  yield  in 
carbon-dioxide,  so  effective  as  normal  respiration. 

.Exir  all  of  the  higher   plants   prolonged   intramolecular 
respiration  is  jmpossible.    To  what  this  is  due  is  not  wholly 

*  For  directions  for  laboratory  and  demonstration  experiments  on  this 
subject  consult  Darwin  &  Acton's  Practical  Physiology  of  Plants ;  Moor's 
translation,  under  similar  title,  of  Detmer's  Pflanzenphysiologisches  Prak- 
tikum,  or  the  special  papers  on  the  subject  cited  in  Pfeffer's  Handbuch  der 
Pflanzenphysiologie,  and  its  English  translation,  and  also  in  Ganong's  and 
MacDougal's  manuals. 


28  PLANT  PHYSIOLOGY 

clear.  The  substances  first  broken  up  in  intramolecular 
respiration  are  the  same  as  in  normal  respiration,  namely, 
the  sugars,  starches  ( after  conversion  into  sugars ) ,  and  the 
fats  and  oils;  but  later  the  proteid  substances  enclosed  in 
the  cell,  and  finally  the  living  substance  itself,  are  decom- 
posed to  supply  needed  energy.  Whether  the  cessation  of 
intramolecular  respiration  in  experiments  upon  higher 
plants,  and  the  consequent  death  of  the  organism,  are  due 
to  the  destruction  of  a  part  of  the  living  substance,  or 
whether  they  are  due  to  the  production  in  the  cells  of 
poisonous  substances,  cannot  now  be  positively  asserted. 
Certain  it  is  that  for  higher  organisms  intramolecular 
respiration  is  a  function  very  limited  in  importance,  taking 
place  only  under  the  stress  of  continued  need  of  energy,  in 
the  absence  of  an  adequate  supply  of  free  oxygen,  and 
capable  of  being  maintained  for  comparatively  brief  periods 
only.  Like  normal  respiration,  it  is  carried  on  solely  by  the 
living  protoplasm,  more  or  less  actively  according  to  the 
greater  or  lesser  activity  of  the  protoplasm ;  the  substances 
decomposed  are  like  those  oxidized  in  normal  respiration 
and  differ  in  different  species  of  plants;  tjie  products  differ 
according  to  the  plant,  the  conditions  unfter  which  it  acts, 
and  the  substances  acted  upon. 

;  Besides  carbon-dioxide,  alcohol  may  be  produced  in  con- 
siderable amount — suggesting  the  connection  between  fer- 
mentation and  intramolecular  respiration — and  also  organic, 
acids,  together  with  small  amounts  of  many  other  com- 
pounds. In  germinating  peas,  the  alcohol  produced  may 
equal  as  much  as  5%  the  weight  of  the  moist  seeds,*  enough 
to  give  some  support  to  the  hypothesis  suggested  above 
that  it  may  be  the  accumulation  of  the  poisonous  pro- 
ducts of  intramolecular  respiration  which,  as  we  shall  see  is 
the  case  in  fermentation  (pp.  30-37),  brings  about  the 
cessation  of  respiration  and  the  death  of  the  organism. 

Between  those  plants  for  which  aerobic  respiration  is  in- 
dispensable to  normally  active  life,  and  for  which  anaerobic 
respiration  is  only  a  means  of  maintaining  life  over  un- 

*  Godlewski.  In  the  Anzeiger  der  Krakauer  Akademie,  Juli,  1897.  Re- 
viewed in  Botanische  Zeitung,  Sept.  16,  1897. 


RESPIRATION  29 

favorable  periods,  and  those  for  which  anaerobic  respiration 
is  similarly  and  equally  indispensable,  there  are  all  connect- 
ing stages.  These  are  found  among  the  lower  plants,  es- 
pecially among  the  fungi ;  but,  as  before  stated,  in  all  large 
multicellular  organisms,  especially  among  animals,  there  are 
probably  cells,  lying  deep  in  the  tissues,  which  are  forced 
by  the  positions  they  occupy  to  supply  themselves  with 
needed  kinetic  energy  by  the  same  means  as  the  anaerobic 
organisms,  namely,  by  decomposing  the  complex  compounds 
which  they  contain.  There  are,  then,  cells  as  well  as  or- 
ganisms which  are  obligate  aerobic,  facultative  aerobic,  and 
obligate  anaerobic.  The  obligate  anaerobic  cells  and  or- 
ganisms live  where  the  access  of  free  oxygen  is  impossible  or 
difficult :  for  instance,  deep  in  living  tissues,  either  as  com- 
ponent parts  of  these  tissues,  or  as  parasites  and  sapro- 
phytes therein ;  in  the  deeper  layers  of  compact  soils,  in  the 
mud  of  swamps  and  marshes,  and  in  the  ooze  below  bodies 
of  comparatively  still  water,  fresh  and  salt ;  indeed,  wherever 
there  are  proper  food,  proper  temperature,  and  proper 
freedom. 

As  in  aerobic  so  also  in  anaerobic  respiration,  other 
processes  take  place  simultaneously  with  it.  These,  if  not 
directly  caused  by  respiration,  are  at  all  events  main- 
tained by  the  energy  liberated  in  it,  and  are  so  closely 
connected  with  it  that  to  distinguish  betwreen  the  chemical 
products  of  respiration  and  those  of  the  processes  accom- 
panying it,  is  a  matter  exceedingly  difficult  and  not  yet 
fully  accomplished.  Fermentation,  decay,  and  disease  at 
least  accompany,  if  they  are  not  actually  a  part  of,  the 
respiratory  processes  of  certain  low  plants.  Anaerobic  respi- 
ration, as  well  as  aerobic,  is  a  function  of  the  living  pro- 
toplasm, which  acts  upon  substances  enclosed  within  its  own 
body,  producing  simpler  substances,  of  which  some  remain 
in  the  respiring  cell  while  others  diffuse  out  of  it.  Some  of 
the  latter  are  entirely  inactive  chemically,  like  carbon-diox- 
ide and  alcohol;  others  may  act  upon  the  substances  out- 
side of  the  cell.  In  the  higher  animals  and  plants  the  enzyms 
( like  pepsin,  diastase,  etc. )  are  produced  in  connection  with 
the  process  of  nutrition,  converting  the  substances  upon 


30  PLANT  PHYSIOLOGY 

which  they  act  into  available  food  compounds ;  but  it  is  also 
certain  that,  among  the  enzyms  produced,  there  are  some 
which  bring  about  such  changes  in  the  surrounding  sub- 
stances that  these  become  available  as  sources  of  kinetic 
energy.  For  example,  the  diastase  formed  in  the  germinat- 
ing seed,  which  dissolves  the  starch  deposited  as  a  reserve 
food  in  the  seed,  converting  it  into  sugar,  makes  the  reserve 
food  available  for  at  least  three  purposes ;  first,  for  the  con- 
struction of  nitrogenous  compounds  ( amides  and  proteids )  ; 
second,  for  the  formation  of  cell-wall  (cellulose)  ;  and  third, 
for  the  liberation  of  energy  by  aerobic  respiration.  The  pro- 
duction and  action  of  this  enzym  furnishes  material  for 
respiration  as  well  as  for  nutrition.  The  enzyms  formed  by 
the  lower  plants  are  also  useful  in  more  than  one  way,  and 
in  many  cases  one  of  these  uses  is  undoubtedly  the  con- 
version of  irrespirable  into  respirable  substances. 

FERMENTATIONS 

In  the  group  of  processes  which  are  commonly  called  fer- 
mentations ( alcoholic,  lactic,  butyric,  etc. ) ,  the  physiologist 
must  study  both  respiration  and  nutrition.  A  thorough 
understanding  of  the  physiology  of  fermentation*  is  impos- 
sible without  previous  thorough  knowledge  of  the  chemistry 
of  fermentation.  As  this  chemical  knowledge  is  still  incom- 
plete, it  is  possible  to  form  only  general  ideas  concerning 
fermentation  and  the  allied  processes  of  decay  and  disease. 
The  distinction  usually  made  between  fermentation  and 
decay  is  this,  that  in  fermentation  there  is  evolution  of  gas, 
whereas,  in  decay,  there  is  little  or  none — an  artificial  and 
even  misleading  distinction.  Even  the  attempted  distinc- 
tion between  disease  on  the  one  hand,  and  fermentation  and 
decay  on  the  other,  is  anything  but  fundamental;  for  dis- 
ease, so  far  as  it  is  the  result  of  the  activity  of  foreign 
organisms,  is  nothing  more  than  decay  or  fermentation  car- 
ried on  by  those  foreign  organisms  upon  or  in  the  living 
body  of  the  host. 

*  On  the  subject  of  fermentation  the  student  may  well  consult  J.  Rey- 
nolds Green's  "The  Soluble  Ferments  and  Fermentation."  Cambridge, 
England,  1901.  In  this  book  there  is  a  very  useful  bibliography. 


RESPIRATION  31 

Although  at  present  unable  to  determine  the  respective 
parts  taken  by  respiration  and  nutrition  in  fermentation, 
decay,  and  disease,  let  us  briefly  consider  some  examples. 
The  fermentation  most  studied  because  the  most  useful  is 
alcoholic  fermentation,  by  which  is  understood  the  produc- 
tion of  the  ethyl  alcohol  of  commerce.  The  organisms  which 
produce  most  alcohol  from  a  given  quantity  of  sugar  are 
certain  species  of  yeast  (Saccharomyces  cerevisise,  S.  eltip- 
soidenfi ) ,  but  certain  species  of  Mucor*  ( M.  erectus,  M.  race- 
niosus,  even  M.  mucedo)  and  bacteria  also  form  ethyl  alco- 
hol in  considerable  quantities.  We  have  already  seen  that 
in  the  forced  anaerobic  respiration  of  higher  plants  (fruits 
and  germinating  seeds)  alcohol  is  one  of  the  products.  It 
may  therefore  be  inferred  that,  like  anaerobic  respiration, 
the  production  of  alcohol  in  fermentation  is  not  peculiar  to 
the  plants  most  used  for  the  purpose,  but  that  the  power  to 
produce  it  is  possessed  by  most,  if  not  all,  plants.  In  the 
majority,  however,  this  power  is  undeveloped. 

Not  only  the  amount  of  alcohol  but  also  the  nature  and 
the  amounts  of  the  other  substances  produced,  determine 
the  availability  of  a  plant  for  the  commercial  production  of 
ethyl  alcohol.  The  best  yeasts,  cultivated  under  the  most 
favorable  conditions,  convert  about  6%  of  the  sugar  used 
into  other  compounds  than  alcohol,  e.  g\  glycerine,  succinic 
acid,  higher  alcohols,  ethereal  compounds  (to  which  the 
''bouquet"  of  wines  is  chiefly  due),  etc.  Under  less  favor- 
able conditions,  with  poorer  yeasts,  and  especially  with 
mixtures  of  yeasts  and  bacteria,  the  amounts  and  kinds  of 
undesirable  by-products  increase.  The  amount  of  alcohol 
produced  by  the  bacteria  commonly  associated  with  beer, 
bread,  wine,  and  other  yeasts  used  in  the  arts  in  this  coun- 
try, is  slight  in  comparison  with  their  other  products ;  and 
it  is  quite  as  much  the  kinds  and  amounts  of  these  other 

*  See  E.  Chr.  Hansen.  Meddelelsor  fra  Carlsberg  Laboratoriet,  Bd.  I., 
etc.  Untersuchungen  a.  d.  Praxis  der  Gahrungsmdustrie.  Jorgensen.  Die 
mikroorganismen  der  Gahnmgsmdustrie,  3te  Aufl.,  Berlin,  1892.  E.  Got- 
schlich.  Gahrungserregung ;  in  Fliigge's  Mikroorganiemen,  3te  Aufl.,  Leip- 
zig, 1896,  Bd.  I.,  pp.  219-270.  Lalar.  Technical  Mycology,  tranel.  by 
Salter.  London  and  Philadelphia,  1898. 


32  PLANT  PHYSIOLOGY 

products,  as  the  quality  of  the  fermentable  mixture  into 
which  they  are  introduced,  which  give  the  peculiar  color, 
texture,  flavor,  and  odor  of  the  different  brands.  Unless  the 
mixture  to  be  fermented  is  first  sterilized  and  then  inocu- 
lated with  pure  yeast  exclusively,  every  bottle  of  wine,  beer, 
or  other  liquor,  and  every  loaf  of  bread,  represents  the  com- 
bined action  of  yeasts  and  bacteria.  It  contains  the  pro- 
ducts, alcoholic  and  other,  so  far  as  they  are  not  driven 
off  or  decomposed  in  the  process  of  manufacture,  of  all 
the  organisms  in  the  fermenting  substance.  Preliminary 
sterilization,  the  inoculation  with  pure  cultures  of  yeasts, 
and  fermenting  under  uniform  conditions,  are  the  se- 
cret of  the  uniformly  good  quality  of  the  well-known  Ger- 
man beers  and  wines.  In  America  such  precautions  are 
unusual. 

The  substances  acted  upon  bear  a  very  definite  relation  to 
the  active  organisms.  Not  all  substances  are  fermentable, 
not  even  all  substances  of  the  same  proportional  composi- 
tion. Though  there  may  be  in  one  substance  the  same,  and 
the  same  number  of,  atoms,  these  may  be  so  differently  ar- 
ranged that  the  fermenting  organism  can  act  upon  the  one 
but  cannot  upon  the  other.  The  yeasts  are  able  to  ferment 
only  a  limited  number  of  sugars,  which  are  in  the  main 
characteristic  of  the  different  species.*  Some  of  the  yeasts 
secrete  enzymsf  which  convert  starch  and  cellulose  into 
sugar.  Thus  ordinary  bread  yeast,  unable  to  ferment  the 
starch  with  which  it  may  be  brought  into  contact,  secretes 
an  enzym  which  converts  the  starch  into  sugar.  Further- 
more, cane-sugar  cannot  be  directly  fermented,  and  when 
present  in  dough,  etc.,  it  must  first  be  acted  upon  by  an 
'enzym,  "inverted."  The  resulting  sugar,  differing  only  in 
the  arrangement,  not  in  the  number  or  kind,  of  atoms,  is 
directly  fermentable.  These  chemical  changes  can  be  sug- 
gested, though,  on  account  of  the  by-products  already 

*  For  tables  of  these  see  Fliigge's  Mikroorganismen,  3te  Auflage,  Bd.  I., 
p.  204,  and  the  papers  by  Emil '  Fischer  and  collaborators  in  Ber.  d. 
Deutsch.  Chem.  Gesellschaft  for  the  last  few  years. 

t  Effront,  J.  Enzyms  and  their  applications.  English  translation  by 
Prescott,  S.  C.  1902. 


RESPIRATION  33 

alluded  to,  not  accurately  represented,    by   the  following 
reactions : 

C8  H10  05  +  Enzym  +  Aq  =  C6  Hlt  O6 

(Starch)        (Diastase?)  (Dextrose,  etc.) 

C6  H12  O6  +  Yeast  +  Aq  =  2  CO2  +  2  C2  H6  O 

By  what  has  already  been  said  we  are  led  to  infer  that 
organisms  capable  of  such  decompositions  as  the  above  sup- 
ply themselves  by  this  means  with  kinetic  energy,  and  can 
flourish  although  little  or  no  energy  may  come  from  other 
sources.  In  fact,  the  yeasts  are  facultative  anaerobic  organ- 
isms, demanding  free  oxygen  only  when  inadequately  sup- 
plied with  the  appropriate  sugar.  The  actual  fermentation 
is  often  only  one  of  a  series  of  processes,  first  made  possible 
by  the  inverting  action  of  an  enzym  upon  some  not  directly 
fermentable  sugar,  and  followed  by  other  processes  for  which 
the  fermentation  supplies  the  necessary  energy.  Buchner 
and  his  collaborators*  have  demonstrated  within  the  last 
few  years  that  the  alcoholic  fermentation  by  yeast  is  ac- 
complished by  an  enzym  (zymase)  which  splits  the  sugar 
into  carbon-dioxide  and  alcohol,  releasing  energy  which  the 
yeast  cells  use.  This  discovery  furnishes  the  ground  for  the 
hypothesis  that  all  respiration,  not  merely  alcoholic  fer- 
mentation, is  carried  on  by  the  living  cell  through  the 
agency  of  enzyms  (see  p.  19). 

The  yeasts  are  more  profitable  for  the  commercial  produc- 
tion of  alcohol,  not  only  because  of  the  larger  amount  of 
alcohol  which  they  will  produce  from  a  given  quantity  of 
sugar,  but  also  because  they  can  continue  their  rate  of  pro- 
duction in  the  presence  of  a  larger  amount  of  alcohol  than 
most  organisms  can  survive.  The  species  of  Mucor  studied 
by  Hansenf  produce  only  from  3%  to  8%  of  alcohol,  whereas 
the  most  active  species  of  yeast  can  produce  14%,  although 
their  activity  decreases  with  12%.  It  is  obviously  impossible, 
therefore,  to  produce  by  means  of  yeasts  alone  any  liquor 
containing  more  than  14%  of  alcohol.  Those  wines  which 

*  Buchner,  E.    Alcoholische  Gahning  ohne  Hefezellen.    Ber.  d.  Deutsch. 
Chem.  Gesellsch.,  1897  and  years  following, 
t  Loc.  cit.    Bd.  II.  p.  160. 
3 


34  PLANT  PHYSIOLOGY 


contain  more  (like  port,  with  17%  or  more)  must  be  "forti- 
fied," tha^BL  alcohol  must  be  added  to  them;  while  whis- 
key, brandy,  \tc.,  are  still  more  artificial,  being  distilled 
liquors. 

The  souring  of  milk,  the  so-called  lactic-acid  fermentation, 
is  due  to  jfa  activity  of  a  great  number  of  aerobic  and 
anaerobic  fflranisms,  especially  bacteria.  These  ferment  the 
sugars  founoin  solution  in  milk,  first  inverting  the  sugar  if 
necessary,  then  splitting  it  mainly  into  lactic  acid,  thus — 

C6  Hw  0.  -  2  C,  H.  03 

but  producing  also,  in  proportions  characteristic  of  the  spe- 
cies, acetic,  formic,  and  other  organic  acids,  and  carbon- 
dioxide.  Since  these  organisms  can  be  active  in  the  presence 
of  only  a  comparatively  small  amount  of  free  acid,  only  a 
small  part  of  the  available  sugar  is,  under  natural  condi- 
tions, fermented  by  them.  If,  however,  the  free  acid  be 
neutralized  by  the  addition  of  calcic-carbonate  to  the  milk, 
fermentation  will  be  resumed  and  continued  until  again  an 
excess  of  free  acid  accumulates.  The  addition  of  lime-water 
(a  dilute  solution  of  calcic  hydrate)  to  the  milk  given  to 
infants  is  advantageous  because  it  neutralizes  the  traces  of 
free  acid  which  may  have  been  formed  in  the  milk. 

Butter  becomes  rancid  when  the  obligate  aerobic  and  the 
facultative  or  obligate  anaerobic  bacteria  invariably  present 
in  butter  succeed  in  decomposing  the  dextrose  and  other 
non-nitrogenous  substances,  possibly  also  some  of  the  pro- 
teids,  into  a  number  of  simpler  compounds  of  which  butyric 
acid  is  the  most  abundant  and  characteristic.  The  by- 
products, gaseous  and  other,  odorous  or  not,  vary  with  the 
organisms  accomplishing  the  fermentation,  while  the  sub- 
stances acted  upon,  the  course  of  the  reactions,  and  the 
activity  of  the  fermentation  vary  also  with  the  conditions. 

It  must  not  be  inferred  that  all  so-called  fermentations  are 
the  result  of  splitting  the  compounds  acted  upon  into  sim- 
pler ones.  The  acetic  acid  fermentation,  for  example,  is  an 
oxidation  carried  on  by  aerobic  bacteria  (two  species,  ac- 
cording to  Hansen,*  Bacterium  aceti,  B.  Posteu rianu m), 

*  Hansen.    Loc.  cit. 


RESPIRATION  35 

although,  as  we  have  seen,  acetic  acid  is  a  common  by- 
product in  anaerobic  fermentations.  Acetic  acid  is  produced 
on  a  commercial  scale  either  by  destructive  distillation  of 
wood,  or  by  the  slow  oxidation  of  dilute  solutions  of  alco- 
hol in  the  presence  of  free  oxygen  (L  e.  in  the  air)  by  living 
organisms.  The  optimum  temperature  for  these  (20-30°  C.) 
is  lower  than  the  optimum  for  alcoholic  (25-30  C.),  butyric 
(35°  C.),  and  lactic  (45-50°  C.)  fermentations,  and  they 
can  withstand  a  decidedly  higher  amount  (8-14%)  of  free 
acid  than  most  other  organisms.*  So  long  as  the  acetic 
bacteria  are  adequately  supplied  with  alcohol  for  respira- 
tion, they  continue  to  form  acetic  acid  to  the  amount  above 
indicated ;  but  if  the  supply  of  alcohol  is  insufficient,  they 
oxidize  some  of  the  acetic  acid,  setting  free  carbon-dioxide, 
which  is  not  produced  under  favorable  conditions. 

Besides  these  fermentations,  which  are  carried  on  in  solu- 
tions by  specific  organisms,  there  are  similar  processes  car- 
ried on  in  the  living  body  by  specific  parasitic  organisms, 
mainly  bacteria,  which  liberate  the  energy  and  supply 
themselves  with  the  food  which  they  need  by  attacking 
either  the  living  substance  of  the  body  itself  or  the  lifeless 
food-substances  enclosed  within  its  cells.  The  main  and  the 
by-products  of  these  parasitic  organisms  cause  the  system- 
atic disturbances  of  the  functions  of  the  host  organism, 
which  are  characteristic  of  the  different  forms  of  disease. 
So,  for  instance,  the  bacillus  of  diphtheria  (Bacillus  diph- 
therite,  K-L.)  causes  decompositions  in  the  limited  area  of 
the  mucous  membrane  of  the  throat,  which  it  commonly 
attacks,  and  the  products  of  these  decompositions,  poisons, 
diffusing  through  the  body  of  the  host,  are  the  direct  causes 
of  the  disease  rather  than  the  presence,  merely  locally  irri- 
tating, of  the  parasitic  organisms.  This  discovery,  that  the 
poisonous  products  of  the  specific  bacteria  of  disease,  and 
not  the  bacteria  themselves,  are  the  direct  causes  of  disease, 
has  opened  a  new  chapter  in  the  science  of  medicine ;  but  so 
long  as  our  knowledge  of  the  chemistry  of  these  products, 

*  Citromyces,  according  to  Wehmer  (Beitrage  z.  Kenntnis  einheimischer 
Pilze.  I),  can  withstand  successfully  at  least  20%  of  the  citric  acid  which 
it  forms  from  sugars. 


36  PLANT  PHYSIOLOGY 

and  of  the  processes  which  result  in  their  formation,  lags 
behind  our  knowledge  of  the  organisms  themselves,  the 
treatment  of  the  germ-diseases  must  continue  to  be  largely 
a  groping  in  the  dark. 

In  addition  to  those  fermentations  or  decompositions  ac- 
complished in  the  living  body  or  in  lifeless  substances  by 
distinct  species  of  organisms,  there  are  other  equally  pro- 
found chemical  disturbances  which  are  due  to  the  combined 
activities  of  organisms  of  more  than  one  species.  In  the 
comparatively  simple  case  of  the  so-called  nitrogen  bacteria 
one  species  decomposes  and  converts  ammonia  salts  into 
those  of  nitrous  acid,  and  another  these  nitrous  salts  into 
nitric.  Many  other  decompositions  taking  place  in  nature 
are  the  result  of  the  co-operation  of  several  or  many  species 
of  organisms,  some  anaerobic,  others  aerobic,  the  former 
preparing  the  way  for  the  latter.  The  final  products  and  all 
the  intermediate  ones  will  vary  according  to  the  organisms 
co-operating.  The  simplest,  though  not  necessarily  the 
mildest,  diseases  are  due  to  the  activity  of  single  species  oi 
organisms.  The  simplest  fermentations  are  the  same.  Both 
are  decompositions,  with  or  without  oxygen.  The  more 
complicated  diseases  are  those  due  to  mixed  infections, 
in  which  two  or  more  species  act  together.  The  one  species 
makes  life  easier  for  the  other,  the  products  of  the  one  fur- 
nish food  or  the  sources  of  energy  for  the  other,  the  main 
or  the  by-products  of  both  supplement  each  other  by  weak- 
ening the  host.  Typhoid-malaria,  and  even  the  mild  pro- 
tective disease  which  is  the  result  of  vaccination,  are  mixed 
infections  due  to  the  co-operation  of  two  or  more  ( probably 
three  or  four )  species,  no  one  of  which  alone  is  able  to  pro- 
duce the  disease. 

We  have  then  in  nature  a  series  of  chemical  decompositions 
accomplished  by  living  organisms,  animals  and  plants,  for 
the  liberation  of  needed  kinetic  energy,  and  resulting  in  suc- 
cessive simplifications  from  the  most  complex  compounds  to 
the  simplest — carbon-dioxide,  ammonia,  and  water.  The 
normal  or  aerobic  respiration  of  all  higher  and  most  lower 
organisms  results  in  the  direct  destruction  by  oxidation  of 
the  most  complex  compounds.  Anaerobic  or  intramolecular 


RESPIRATION  37 

respiration,  fermentation,  decay,  and  disease,  attacking  the 
same  complex  compounds  first,  result  in  the  formation  of 
the  same  simple  ones  finally.  This  is  done  only  slowly, 
through  many  stages,  with  the  same  total  of  released 
energy,  but  with  far  less  energy  available  for  the  individual 
organisms  in  the  long  chain.  Thus  the  energy  stored  in  the 
compounds  elaborated  by  living  organisms  is  set  free  again 
before  and  after  their  death,  either  by  their  own  activity  or 
by  the  activity  of  others.  This  energy  becomes  available 
for  the  construction  of  new  energy-storing  compounds,  for 
work  of  the  utmost  physiological  variety. 

From  the  foregoing  discussion  it  is  evident  that  respira- 
tion, whether  aerobic  or  anaerobic,  is  a  process  most  inti- 
mately connected  with  the  other  functions  of  the  living 
organism;  a  process  dependent  in  the  first  place  upon  a 
supply  of  respirable  (oxidizable  or  otherwise  decomposable) 
material;  a  process  which  goes  on,  when  once  respirable 
material  is  adequately  supplied,  actively  or  tardily,  accord- 
ing to  the  activity  of  the  other  functions,  in  other  words, 
according  to  the  demand  for  energy  to  do  work.  In  the 
healthy  organism,  under  natural  conditions,  the  rate  of 
respiration  will  equal  the  demand  for  energy;  it  will  not 
exceed  the  demand,  for  stimulating  respiration  results  in 
stimulating  the  other  activities ;  it  will  not  be  less  than  the 
demand,  for  decreasing  respiration  decreases  the  amount  of 
energy  available  for  the  other  activities.  The  rate  of  res- 
piration is  controlled  by  the  living  cell,  and  whatever  in- 
fluences affect  the  total  amount  of  energy  demanded  by  it 
will  also  affect  the  rate  of  respiration,  increasing  or  de- 
pressing the  rate  accordingly. 

Respiration  lias  no  optimum  temperature,  no  optimum 
illumination,  of  its  own:  it  will  increase  or  decrease  with 
changes  in  temperature  and  illumination  only  as  these  affect 
the  demand  for  energy  on  the  part  of  the  organism.  Nutri- 
ent and  poisonous  substances  and  the  amount  of  water  will 
affect  respiration  only  as  they  affect  the  organism.  Injuries* 
cause  a  change  in  the  rate  of  respiration,  but  they  do  so 

*  Richards,  H.  M.  The  respiration  of  wounded  plants.  Annals  of  Bot- 
any, Vol.  X.,  1896. 


38  PLANT  PHYSIOLOGY 

only  by  changing  the  demand  for  energy.  To  repair  an 
injury  or  to  close  a  wound  new  cell-walls  must  be  deposited, 
new  cells  must  be  formed,  more  food  must  be  manufactured, 
or  stored  food  must  be  dissolved,  the  food  must  be  carried 
to  the  seat  of  the  injury  and  used  there,  and  other  kinds  of 
work  must  be  done.  For  all  this  extra  work  extra  energy 
must  be  supplied,  and  hence  the  rate  of  respiration  must  be 
increased.  This  increased  rate  shows  itself  in  plants  as  in 
animals  by  the  increased  production  of  carbon-dioxide  and 
by  a  rise  in  temperature  ( wound-fever ) .  In  parts  and  in  or- 
ganisms most  rapidly  growing  the  rate  of  respiration  will  be 
high,  but  growth  is  only  one  function  and  when  it  decreases 
other  functions  may  become  more  active,  keeping  the  de- 
mand for  energy  uniform,  and  hence  the  rate  of  respiration 
will  not  necessarily  decrease  with  a  decreasing  growth- 
rate. 

The  rate  of  respiration  varies  with  the  total  demand  for 
energy,  not  with  any  one  function.  For  instance,  the  rate 
of  respiration  and  consequently  the  liberation  of  energy 
(heat)  reach  their  maximum  in  the  spathes  of  Aroids*  after 
the  parts  have  passed  the  period  of  most  rapid  growth ;  for, 
though  the  growth-rate  diminishes,  the  various  processes 
concerned  in  and  stimulated  by  fertilization  demand  an 
equal  or  even  greater  amount  of  energy. 

In  normal  respiration  the  volumes  of  oxygen  fixed  and 
of  carbon-dioxide  evolved  are  approximately  equal.  When 
this  ratio  is  not  obtained  in  an  experiment,  the  difference  is 
to  be  accounted  for  in  one  or  more  of  the  following  ways. 
First,  the  ratio  will  vary  with  the  amount  of  oxygen  en- 
closed in  the  body  of  the  organism  at  the  beginning  of  the 
experiment.  This  oxygen  will  be  used  before  more  can  be 
absorbed.  Second,  only  so  much  of  the  carbon-dioxide 
evolved  (exhaled)  can  be  measured  as  is  not  absorbed,  or 
does  not  remain  enclosed,  by  the  tissues,  or,  in  the  case  of 
water-plants,  is  not  dissolved  in  the  water.  Third,  other 
substances  than  carbon-dioxide  and  water  may  be  formed 
in  respiration  ( e.  g.  organic  acids,  alcohol,  etc. )  and  there- 
fore the  different  amounts  of  these  other  products  in  differ- 

*  See  Pfeffer,  Pflanzenphysiologie,  Bd.  II.,  p.  404,  of  the  first  edition. 


RESPIRATION  39 

ent  organisms  will  evidently  affect  the  ratio  between  oxygen 
absorbed  and  carbon-dioxide  given  off.  The  ratio  will  be 
changed  either  by  the  greater  absorption  of  oxygen  for  the 
oxidation  of  organic  acids  or  by  the  smaller  amount  of 
carbon-dioxide  produced  when  such  incompletely  oxidized 
by-products  as  alcohol  are  formed.  Finally,  every  factor  in 
the  environment  of  the  organism,  its  food,  the  influence  of 
its  neighbors,  the  weather,  etc.,  will  affect  the  ratio  more  or 
less  by  affecting  the  organism  itself. 

For  the  reasons  made  clear  in  the  foregoing  pages,  the 
amount  of  carbon-dioxide  exhaled  is  by  no  means  an  ac- 
curate index  of  the  rate  of  respiration  and  of  the  amount 
of  energy  liberated,  but  taking  this  substance  only,  we  may 
still  make  some  interesting  comparisons.  For  instance — * 
nan  exhales  in  24  hours  CO2,  equal  to  1.2%  of  his  body- 

eight;  moulds  exhale  in  24  hours  CO2,  equal  to  6.0%  of 
their  body- weight ;  active  bacteria  exhale  in  24  hours  CO., 
equal  to  200.0%  of  their  body- weight.  Organisms  which, 
like  man,  have  comparatively  feeble  respiratory  power, 
must  either,  as  he  does,  supply  themselves  with  energy  by 
external  means  or  limit  their  activities  to  what  they  can 
themselves  produce.  ~£lvjng  birds  produce  much  larger 
amounts  of  carbon-dioxide  and  hence  of  energy  than  man, 
and  upon  the  latter  depends  their  abilit}^  to  fly.  The  very 
important  part  taken  by  the  moulds  and  bacteria  in 
Nature  is  accounted  for  by  their  being  able  to  supply 
themselves  with  such  tremendous  amounts  of  energy. 

SUMMARY 

Respiration,  controlled  and  accomplished  by  living  organ- 
isms, produces  disturbances  of  equilibrium  in  the  living 
substances  of  the  organism ;  these  disturbances  give  im- 
pulses to  further  molecular  and  atomic  movements  and  to 
further  vital  acti vit ies-rrespir ation  traqffif flTin »  latent  (po- 
tential) to  active  (kinetic)  energy,  and  reduces  the  amounts 
of  carbohydrates  and  fats,  to  a  less  degree  also  proto- 
plasmic substances,  in  the  body. 

*  Pfeffer.   Pflanzenphysiologie,  Bd.  I.,  p.  526.  Engl.  transl.,vol.  I.,  p.  522. 


CHAPTER    III 

NUTRITION 

FROM  the  preceding  chapter  we  have  learned  that  respira- 
tion is  a  destructive  process  consisting  either  in  the  break- 
ing up  of  complex  compounds  into  simpler  ones,  or  in  the 
physiological  oxidation  of  various  combustible  substances. 
With  the  important  exception  of  the  nitrogen,  sulphur,  and 
iron  bacteria,  which  derive  their  energy  from  simple  com- 
pounds, all  living  organisms  depend  for  their  chief  supply  of 
energy  upon  complex  compounds  existing  in  nature  only  as 
the  result  of  the  constructive  activities  of  living  organisms. 
In  the  last  chapter  we  assumed  the  presence  of  these  com- 
plex compounds,  examining  only  the  means  of  deriving 
energy  from  them. 

Energy  is  needed  for  construction,  to  do  work.  Only  so 
much  energy  can  be  liberated  by  complete  combustion  or 
complete  decomposition  as  was  employed  in  construction. 
Theoretically  just  as  much  energy  should  be  liberated  in  the 
combustion  of  a  starch  grain  as  was  needed  to  make  it,  but 
the  cell  is  not  a  perfect  machine;  not  all  the  energy  or 
power  used  goes  into  the  finished  product,  some  is  expended 
in  overcoming  the  internal  resistance  of  the  machine,  some 
is  radiated,  or  "lost"  in  other  ways.  There  is  waste  of 
energy  and  of  material  in  every  machine,  the  product  does 
not  represent  the  total  expenditure  of  material  and  energy. 
As  it  costs  a  certain  amount  of  energy  to  keep  an  engine 
going  without  its  doing  any  other  work,  and  a  larger 
amount  to  make  it  do  work  as  well  as  go,  so  it  costs  a 
certain  amount  of  energy  to  keep  an  organism  alive  and 
more  to  make  it  do  anything.  The  sum  of  the  energy  ex- 
pended in  making  the  engine  go,  plus  the  amount  expended 
in  making  it  do  work,  equals  the  amount  of  energy  which 
must  be  developed  to  run  it,  if  there  is  no  loss  by  radia- 


NUTRITION  41 

tion,  etc.  So  also  the  sum  of  the  energy  necessary  to  living, 
plus  the  energy  necessary  to  action,  equals  the  amount  de- 
veloped, provided  there  is  no  loss  by  radiation,  etc.  In  the 
living  organism  this  loss  is  less  than  in  the  machine;  the 
organism  is  economical  to  a  high  degree. 

With  the  exception  of  the  bacteria  mentioned  (p.  40 ),  all 
living  organisms  must  have  complex  compounds,  elaborated 
either  by  themselves  or  by  other  living  organisms,  from 
which  to  derive  the  energy  which  they  need  to  continue 
living  and  to  carry  on  their  various  forms  of  activity. 
These  complex  compounds  are  elaborated  from  simple  ones. 
Owing  to  the  discrepancy  existing,  even  in  the  most  eco- 
nomical organism,  between  the  amount  of  kinetic  energy 
required  to  elaborate  a  complex  compound  and  the  amount 
of  kinetic  energy  which  can  be  liberated  from  it,  the  organ- 
ism cannot  supply  itself  by  respiration  alone  with  sufficient 
energy  to  elaborate  all  the  compounds  it  needs.  It  must 
acquire  energy  from  outside  of  itself.  This  the  majority  of 
organisms  do  by  taking  into  their  bodies  more  food  than 
they  incorporate.  Some  of  this  food  is  destroyed  to  furnish 
energy  for  the  elaboration,  assimilation,  and  incorporation 
of  the  remainder. 

Obviously  this  system  cannot  prevail  throughout  the 
whole  series  of  living  organisms.  Some  must  obtain  energy 
from  a  source  entirely  independent  of  organic  life.  This 
source  is  the  sun,  from  which  energy  comes  to  the  earth  in 
the  form  of  light.  As  will  be  shown  more  plainly  later  ( pp. 
55,  56),  the  sunlight  is  composed  of  radiant  energy  of  at 
least  three  distinct  sorts,  of  so-called  heat,  light,  and  chemi- 
cal rays.  Of  these  the  light  rays  are  the  ones  most  used  in 
making  good  the  losses  of  energy  due  to  living  organisms 
not  working  with  absolute  economy.  These  rays  are  used 
to  combine  two  of  the  food-materials  concerned  in  the 
process  of  nutrition. 

Nutrition  consists  in,  1st,  the  absolution,  and  2d,  the 
combination  of  food-materials,  and  3d,  the  assimilation, 
and  4th,  the  incorporation  of  foods.  Nutrition  furnishes 
the  materials  for  (a)  supplying  energy. (b)  the  construc- 
tion of  new  parts,  (c)  the  repair  of  worn  parts.  Every 


42  PLANT  PHYSIOLOGY 

individual  must  sooner  or  later  nourish  itself,  because  food 
enough  for  continued  living  is  not  given  to  it  by  its  parents 
in  the  seed,  spore,  or  egg,  or  later. 

THE  FOOD-MATERIALS 

The  food-materials  of  all  organisms  are  fundamentally 
alike.  This  conclusion  may  be  formed  from  the  observation 
that  all  animals  are  directly  or  indirectly  dependent  upon 
plants  for  food— the  Carnivora  preying  upon  the  Herbivora, 
the  Herbivora  devouring  plants.  It  might  be  suspected  that 
the  nitrogen,  sulphur,  and  iron  bacteria,  peculiar  in  their 
sources  of  energy,  might  be  peculiar  in  the  foods  used  for 
the  construction  of  their  bodies.  Chemical  analysis  of  the 
bodies  of  all  organisms,  especially  of  the  protoplasmic  parts, 
shows  that  there  are  no  exceptions.  The  conclusion  above 
stated  is  further  confirmed  by  culture  experiments  in  which 
food-materials  and  foods  of  known  composition  are  the  only 
ones  employed. 

Analyses  and  cultures  have  shown  that  of  the  seventy  or 
so  chemical  elements  only  ten  are  absolutely  indispensable 
as  food  constituents,  namely,  carbon,  hydrogen,  oxygen, 
nitrogen,  sulphur,  phosphorus,  potassium,  calcium,  magne- 
sium, and  iron.  Two  other  elements  are  invariably  found  in 
analyses  of  the  bodies  of  animals  and  plants,  namely,  sodium 
and  chlorine,  which  are  of  universal  occurrence  combined  as 
common  salt.  Only  with  the  utmost  difficulty  is  this  com- 
pound excluded  from  cultures,  and  there  may  be  doubt 
of  its  ever  having  been  done.  Owing  to  the  universal 
presence  of  common  salt,  rather  than  because  of  its  useful- 
ness, it  occurs  in  all  organisms.  Besides  these  twelve  ele- 
ments, others  occur  in  more  or  less  soluble  compounds  in 
the  bodies  of  organisms.  Some  of  these  may  be  very  useful 
though  not  indispensable.  The  composition  of  the  soil  or 
water  in  which  plants  live,  will  directly  affect  the  composi- 
tion of  their  bodies.  Plants  living  in  soils  containing  large 
quantities  of  copper,  zinc,  arsenic,  aluminum,  or  silica  will 
necessarily  absorb  larger  amounts  of  the  salts  of  these  ele- 
ments than  plants  living  elsewhere.  The  plants  containing 
most  silica — the  Equisetuws,  grasses,  and  sedges — employ 


NUTRITION  43 

this  in  stiffening  their  bodies;  but  they  can  successfully  be 
brought  to  maturity  without  silica,  although  they  will  then 
be  comparatively  soft,  mechanically  weak,  though  physio- 
logically vigorous. 

From  the  few  elements  composing  their  food-materials 
plants  manufacture  foods  of  the  utmost  variety.  These  are 
present  in  the  cells,  either  in  solution  or  deposited  in  solid 
form.  The  foods  may  be  divided  into  two  groups  according 
to  the  presence  or  absence  of  nitrogen  as  a  component  ele- 
ment. The  non-nitrogenous  foods  are  the  ones  first  formed 
and  the  simpler — the  sugars,  starches,  and  oils;  the  nitro- 
genous foods  are  elaborated  from  the  former  and  are  more 
complex — the  amides,  proteids,  nucleines,  etc.  Carbon,  hy- 
drogen, and  oxygen  are  constituent  elements  of  all  the  foods 
in  both  groups,  and  carbon  must  be  considered  the  charac- 
teristic component  element  both  of  living  organisms  and  of 
their  food. 

CARBON 

The  source  of  carbon  for  all  organisms  except  the  nitrogen 
bacteria  and  plants  containing  chlorophyll  or  its  apparent 
equivalent  physiologically,  bacterio-purpurin,  is,  directly  or 
indirectly,  these  color-containing  plants.  Since,  however,  the 
purple-bacteria  play  a  comparatively  unimportant  role  as 
food-producers,  and  their  relations  to  carbon  are  like  those 
of  green  plants,  we  need  not  consider  them.  The  nitrogen 
bacteria  will  be  discussed  on  page  68. 

The  direct  source  of  carbon  for  green  plants,  the  ultimate 
source  for  all  organisms,  is  the  carbon-dioxide  of  the  air. 
This  is  proved  by  the  following  facts :  *  1.  Green  plants 
cultivated  in  an  atmosphere  of  normal  composition,  except 
that  every  trace  of  carbon-dioxide  has  been  removed,  soon 
cease  to  grow  and  finally  die.  2.  Green  plants  cultivated  in 
a  soil  of  otherwise  normal  composition,  but  containing  no 

*  For  details  as  to  experiments  demonstrating  these  facts  consult  Dar- 
win and  Acton's  Practical  Physiology  of  Plants,  Moor's  translation  of 
Detmer's,  and  the  more  recent  "Laboratory  Course  in  Plant  Physiology" 
by  Ganong,  and  " Practical  Text-book  of  Plant  Physiology"  by  Mac- 
Dougal. 


44  PLANT  PHYSIOLOGY 

trace  of  carbon-compounds,  thrive  perfectly.  3.  Green 
plants  cultivated  in  water  holding  in  solution  the  ordinary 
soil  constituents  except  the  carbon  compounds,  also  thrive 
perfectly.  4.  Submersed  water  plants  cultivated  in  water 
containing  all  the  usual  salts,  but  no  trace  of  carbon-diox- 
ide, soon  cease  to  grow  and  presently  die.  The  second  and 
third  of  these  experiments  demonstrate  the  error  in  the  old 
theory  and  in  the  present  popular  belief  that  leaf-mould, 
loam,  manures,  and  fertilizers,  natural  and  artificial,  are 
valuable  because  of  their  carbon  content.  Besides  the 
phosphates  which  they  contain,  it  is  probable  that  the  only 
valuable  constituents  are  the  nitrates. 

Carbon-dioxide  forms  about  five-hundredths  of  one  per 
cent,  of  normally  pure  air  (.05%).  From  this  we  can  calcu- 
late the  percentage  of  carbon  thus  : 

Atomic  weight  of  carbon  =  12        C   =  12 
"  "       "  oxygen  =  16        02  =  32 

C02  =  44 
C         12         3 
~r  44  :=  11 


CO,  =  .05%  of  air. 

C  ==.05  +  fV  =  .0135+ 
.-.  C  ==  .01%  of  air. 

From  figures  given  by  Noll*  we  can  calculate  the  volume  of 
air  necessary  to  furnish  the  carbon  contained    in    a   tree 
weighing  11,000  pounds  dry  and  50%  of  the  dry  weight  of 
which  (5,500  Ibs.  =2,500  Kilos)  is  carbon.    Thus: 
10,000  litres  of  air  contain  5  litres  of  C02 
5       "     "  CO,  weigh  10  gr. 
of  this  C  weighs  $=2  gr. 
.*.  5,000  litres  of  air  contain  1  gr.  of  C. 

"2,500  Kilos  =  2,500,000  grs. 
.-.  2,500,000  x  5,000  =  12,500,000,000  litres 

=  12,500,000  cubic  metres 
=  16,125,000      "      yards 
Hence  16,125,000  cubic  yards  of  air  must  have  their  car- 

*  Text-book  of  Botany,  by  Strasburger,  Noll,  Schenck,  Schimper,  trans- 
lated by  Porter,  p.  196.    New  York,  1898. 


NUTRITION  45 

bon-dioxide  wholly  extracted  to  supply  the  tree.  Does  a 
line  drawn  through  the  periphery  of  a  tree  weighing  11,000 
pounds  when  dry  enclose  this  volume  of  air?  Certainly  not, 
but  this  is  immaterial,  for  the  air  is  constantly  in  motion, 
agitated  by  mechanical  means  and  by  the  winds.  Further- 
more, the  air,  a  mixture  of  gases,  is  subject  to  the  laws 
of  diffusion  of  gases.  The  absorption  or  decomposition  of 
one  volume  of  any  gas  in  a  mixture  will  produce  a  diffusion 
current  of  that  gas  to  make  good  the  loss.  Hence  the  tree, 
whenever  it  is  absorbing  carbon-dioxide,  is  the  centre  to- 
ward which  numberless  diffusion  currents  of  carbon-dioxide 
molecules  are  moving.  The  rapidity  of  their  movement  is 
directly  proportional  to  the  rapidity  of  the  absorption. 

Noll's  figures,  giving  the  amount  of  carbon  contained  in  a 
tree  and  the  volume  of  air  required  to  furnish  this,  may  well 
be  supplemented  by  Brown's  figures  of  the  hourly  absorp- 
tion of  carbon-dioxide.*  According  to  Brown's  determina- 
tions there  is  an  increase  of  one  gram  of  dry  substance 
(mainly  carbohydrates)  to  every  square  meter  of  leaf  sur- 
face of  Csbtalpa,  bignonioides  per  daylight  hour.  To  form 

1  gr.  starch  1.628  gr.  CO2  must  be  absorbed 

1  "     glucose  1.466   "      "  '    "       " 

1  "  saccharose  1.543  "  "  "  " 
From  these  data  1.545  gr.  of  carbon-dioxide  may  be  taken 
as  the  mean  amount  needed  to  form  one  gram  of  carbo- 
hydrate. This  at  normal  temperature  and  pressure  equals 
784  cubic  centimetres  of  carbon-dioxide,  the  volume  of 
carbon-dioxide  absorbed  per  hour  by  each  square  meter  of 
leaf  surface.  If  we  take  into  consideration  the  fact  that 
stomata  occur  only  on  the  under  side  of  Catalpa  leaves, 
and  that  they  occupy  only  1%  of  the  total  leaf  surface,  we 
see  that  the  absorbent  power  of  the  leaf  is  very  great. 

The  daily  absorption  of  such  large  volumes  of  carbon- 
dioxide  from  the  air  suggests  the  query  as  to  the  means  of 
maintaining  the  supply.  We  have  already  seen  that  the 

*  Brown,  H.  T.  The  fixation  of  carbon  by  plants.  Address  to  Chemical 
Section,  Brit.  Assoc.  Adv.  Science,  1899.  Published  in  Nature,  Sept.  14, 
1899. 


46  PLANT  PHYSIOLOGY 

respiration  of  plants  and  animals  returns  to  the  air,  in  the 
form  of  carbon-dioxide,  all  of  the  carbon  contained  in  the 
compounds  physiologically  oxidized.  In  ordinary  combus- 
tion also  the  carbon  of  the  material  burned  returns  to  the 
air  as  carbon-dioxide.  The  subterranean  fires  that  vent 
themselves  in  volcanic  activity  pour  into  the  atmosphere 
very  considerable  quantities  of  carbon-dioxide.  In  these 
three  ways  the  present  proportion  of  caB^on-dioxide  in  the 
atmosphere  is  maintained. 

The  enormous  deposits  of  coal  and  the  accumulations  of 
petroleum  and  natural  gas  suggest  the  hypothesis  that 
in  past  geologic  times  the  earth's  atmosphere  contained 
a  higher  percentage  of  carbon  than  now.  It  is  objected  to 
this  hypothesis  that  the  animals  of  the  Carboniferous 
Period,  the  time  of  greatest  coal  deposition,  could  not  live 
in  air  so  poisonous  as  to  furnish  carbon  enough  for  these 
deposits.  This  objection  does  not  appeal  with  especial  force 
to  physiologists.  It  by  no  means  necessarily  follows  from 
the  inability  of  the  living  relatives  of  the  animals  of  the 
Carboniferous  Period  to  withstand  a  high  percentage  of 
carbon-dioxide  in  the  air,  that  the  earlier  animals  were 
equally  sensit  ye.  Probably  the  animals  of  the  Carbonifer- 
ous Period  could  no  more  exist  under  the  conditions  now 
prevailing,  than  their  nearest  living  relatives  could  survive 
the  conditions  of  the  Carboniferous,  no  matter  how  like  or 
how  different  might  be  the  percentages  of  carbon  in  the  air 
then  and  now.  Furthermore,  there  is  at  least  a  suggestion, 
though  not  evidence,  of  a  difference  between  the  respiratory 
and  other  functions  of  the  organisms  of  the  Carboniferous 
and  those  of  to-day  in  the  fact  that  the  nearest  living  rela- 
tives of  the  plants  of  the  coal  measures  are  the  ones  best 
able  to  survive  high  percentages  of  carbon-dioxide.  By  in- 
creasing the  intensity  of  the  light  as  well  as  the  percentage 
of  carbon-dioxide,  ferns  may  be  made  to  grow  more  rapidly 
than  under  the  normal  conditions  of  the  present  geologic 
age.* 

For  perfect  understanding  of  the  movements  of  gases  we 

*  See  Pfeffer.     Pflanzenphysiologie,  Bd.  I.,  pp.   310-15,  Eng.  Transl.  I., 
p.  332. 


NUTRITION  47 

must  engage  in  the  study  of  physical  chemistry.  For  us, 
however,  who  are  studying  only  the  elements  of  plant- 
physiology,  the  mention  of  a  few  of  the  simplest  principles 
concerned,  will  sufficiently  illuminate  the  subject.  First,  a 
vacuum  cannot  be  produced  adjacent  to  an  unconfined 
volume  of  gas;  the  gas  will  flow,  will  be  drawn,  into  the 
space  whence  gas  is  being  removed  in  the  attempt  to  form  a 
vacuum.  Second,  the  molecules  of  adjacent  and  unconfined 
volumes  of  gases  of  different  compositions  will  move  spon- 
taneously until  the  gases  are  perfectly  mixed.  Third,  the 
same  molecular  movement,  which  is  called  diffusion,  will 
take  place  when  gases  of  different  composition  are  separated 
from  one  another  by  permeable  substances.  Most  animal 
and  vegetable  membranes  are  permeable.  If  one  gas  consist- 
ing of  oxygen  and  carbon-dioxide  is  separated  only  by  a 
permeable  membrane  from  another  gas  consisting  of  oxygen 
alone,  carbon-dioxide  will  pass  from  the  first  through  the 
membrane  into  the  second  until  there  are  equal  proportions 
of  this  gas  on  the  two  sides  of  the  partition,  and  oxygen 
will  pass  from  the  second  into  the  first  until  the  proportion 
of  oxygen  to  carbon-dioxide  is  the  same  in  the  two  volumes. 
If  for  any  reason  ( because  of  higher  or  lower  temperature, 
for  instance)  the  pressures  of  the  two  gases  are  unequal, 
diffusion  of  both  gases  will  continue  until  the  pressure  be- 
comes the  same  on  both  sides  of  the  separating  membrane. 
Diffusion  tends,  then,  to  equalize  the  pressures  and  to  pro- 
duce uniform  composition  in  adjacent  volumes  of  gases  either 
unconfined  or  separated  only  by  permeable  substances. 

Turning  now  to  plants,  we  find  that  the  simplest  plants, 
consisting  either  of  single  cells  or  of  cells  in  filaments  or 
small  masses,  enclose  gases  by  the  permeable  membranes 
bounding  their  cells.  In  higher  plants,  which  consist  of  a 
larger  number  of  cells  in  larger  masses,  the  gases  are  not 
only  enclosed  within  the  cells  by  permeable  membranes,  but 
the  spaces  between  the  cells  also  contain  gases.  These  spaces, 
and  hence  the  volumes  of  gas  which  fill  them,  are  continu- 
ous with  the  unconfined  mixture  of  gases  which  forms  the 
atmosphere.  Any  difference,  therefore,  between  the  compo- 
sition of  the  mixture  of  gases  filling  the  intercellular  spaces 


48  PLANT  PHYSIOLOGY 

and  enclosed  within  the  permeable  membranes  which  bound 
the  cells  of  plants  and  the  atmospheric  air  outside  will 
cause  diffusion.  This  will  tend  to  make  the  composition  the 
same  within  and  without  the  plant.  But  since  every  living 
active  cell  is  constantly  respiring,  taking  in  oxygen  and 
giving  out  carbon-dioxide,  a  difference  in  composition  be- 
tween the  mixture  of  gases  contained  in  the  plant-body  and 
the  atmosphere  is  constantly  brought  about.  As  constant 
will  be  the  diffusion  currents,  one  inward  consisting  of  oxy- 
gen molecules,  one  outward  consisting  of  carbon-dioxide 
molecules.  These  currents,  tending  to  restore  the  uniformity 
in  composition,  but  never  accomplishing  this  so  long  as  the 
plant  respires,  prevent  the  undue  accumulation  of  carbon- 
dioxide  and  maintain  an  adequate  supply  of  oxygen.- 

Only  in  green  plants,  and  in  the  green  cells  of  these,  is 
there  any  even  apparent  exception  to  this  simple  physical 
law.  If,  however,  any  cells  which  liberate  carbon-dioxide  in 
respiration  also  absorb  and  combine  it  again  in  food-manu- 
facture, the  diffusion  currents  to  and  from  these  cells  will 
be  made  up  of  different  molecules  from  those  composing  the 
diffusion  currents  maintained  by  other  cells.  In  the  green 
cells  the  process  and  the  products  of  food-manufacture  are 
the  reverse  of  respiration,  and  the  diffusion  currents  neces- 
sarily correspond.  The  green  cells  respire  as  constantly  and 
at  least  as  actively  as  other  cells  containing  no  chloroplryll ; 
but  while  they  are  manufacturing  food,  they  absorb  and  use 
more  than  all  the  carbon-dioxide  which  they  exhale.  Hence, 
in  experimenting  upon  the  respiration  of  green  plants  or  of 
parts  containing  chlorophyll,  it  is  necessary  to  stop  the 
opposite  and  constructive  process. 

The  absorption  of  carbon-dioxide  gas  is  accomplished  by 
means  of  the  diffusion  currents  of  this  gas  set  and  kept  in 
motion  by  the  combination  of  carbon-dioxide  with  water 
in  the  production  of  food.  The  absorption  takes  place 
through  the  walls  of  all  the  chlorophyll-containing  cells  of 
the  lower  plants.  These  are  all  small  and  for  the  most 
part  aquatics.  In  Fucus  and  other  large  algae  periodically 
exposed  to  the  air,  there  is  a  difference  in  the  absorption  to 
correspond  with  the  division  of  labor  among  the  tissues. 


NUTRITION  49 

Cell-walls  which  are  gelatinized  or  cutinized  and  infiltrated 
with  waxy  matters,  as  are  those  of  the  epidermal  cells  of 
Fucus,  are  less  permeable  to  gases  than  are  cellulose  walls. 
Through  the  cryptostomata  of  Fucus,  therefore,  as  well  as 
through  the  stomata  of  higher  land  plants,  the  inward 
diffusion  of  carbon-dioxide  is  more  rapid  than  through  the 
walls  of  the  epidermal  cells. 

For  land-plants  the  value  of  the  stomata  as  furnishing  the 
gates  through  which  the  exchange  of  gases  takes  place,  has 
been  strikingly  demonstrated  by  Stahl.  *  He  shows  that  by 
smearing  the  stomata-bearing  surface  of  leaves  having 
stomata  on  one  side  only  (usually  the  under  side)  with 
cocoa-butter  or  vaseline  (neither  of  which  is  irritating),  the 
absorption  of  carbon-dioxide,  as  indicated  by  the  amount 
of  starch  formed,  will  be  much  less  than  when  the  gas  can 
diffuse  through  both  the  open  stomata  and  the  more  perme- 
able walls  of  the  epidermal  cells  of  the  stomata-bearing  sur- 
face. If  the  epidermal  cells  of  the  upper  surface  of  a  leaf 
which  has  stomata  only  on  the  under  and  smeared  side  are 
injured  by  a  cut  or  scratch  just  deep  enough  to  penetrate 
the  wall,  the  much  more  rapid  diffusion  of  carbon-dioxide 
through  the  cut  into  the  leaf  than  through  the  walls 
of  uninjured  epidermal  cells  will  be  shown  by  the  greater 
quantity  of  starch  formed  in  the  chlorophyll-containing 
cells  adjacent  to  the  cut.  In  green  land-plants  the  ab- 
sorption of  carbon-dioxide  into  the  intercellular  spaces  is 
accomplished  by  the  free  inward  diffusion  of  the  gas,  a 
diffusion  which  is  controlled  by  the  stomata,  which  can 
open  or  close  as  occasion  demands.  From  these  intercell- 
ular spaces  the  green  cells  absorb  what  carbon-dioxide 
they  need  through  their  permeable  cellulose  walls,  just  as 
the  luwer  algae  absorb  all  their  food-materials  through 
their  permeable  cell-walls. 

As  we  have  already  seen  (p.  45),  the  volume  of  carbon- 
dioxide  absorbed  implies  the  very  considerable  absorbent 
power  of  the  leaf,  but  when  we  recall  how  small  a  propor- 
tion of  the  leaf  surface  (e.  g.  \%  in  Catalpa  bignonioides) 

*  Stahl,    E.    Einige    Versuche    iiber    Transpiration    und    Assimilation. 
Botanische  Zeitung,  Bd.  52,  1894. 
4. 


50  PLANT  PHYSIOLOGY 

can  be  opened  for  the  free  diffusion  of  gases,  we  see  that  the 
absorbent  power  of  leaves  is  still  greater.  According  to 
Brown's  measurements  it  is  fifty  times  that  of  a  strong 
potassic  hydrate  solution  of  equal  surface.*  That  the  dif- 
fusion will  keep  pace  with  this  absorption  is  conceivable 
when  it  is  known  that  the  rate  of  diffusion  through  an 
opening  one  millimeter  in  diameter  is  forty  times  as  fast  as 
through  an  opening  ninety  millimeters  in  diameter.  The 
smaller  the  opening,  within  certain  limits,  the  more  rapid 
the  diffusion  through  it.  Hence  the  movement  of  carbon- 
dioxide  molecules  through  the  stomata  of  an  insolated  leaf 
must  be  very  rapid. 

The  absorption  of  needed  carbon-dioxide  by  green  plants 
is  accomplished,  then,  by  purely  physical  means,  by  diffu- 
sion. The  continued  absorption  of  carbon-dioxide,  and  of 
all  other  food-materials  also,  is  accomplished  by  the  same 
means ;  but  in  order  that  there  should  be  continued  absorp- 
tion, continued  inward  diffusion,  there  must  be  continued 
removal  of  what  is  absorbed.  If  this  were  not  so,  there 
would  presently  result  from  the  diffusion  equal  proportions 
of  the  food-materials  within  and  without  the  plant,  and  the 
diffusion  would  then  cease.  The  construction  of  foods  out 
of  the  food-materials  absorbed  removes  the  food-materials, 
prevents  their  accumulating,  and  continues  the  absorption 
by  maintaining  the  conditions  "for  diffusion. 

If  we  now  examine  the  conditions  and  the  means  necessary 
for  the  elaboration  of  carbon-dioxide,  itself  innutritious 
though  a  food-material,  into  nutritious  matters  or  foods 
proper,  we  shall  at  the  same  time  come  more  clearly  to 
understand  how  it  continues  to  be  absorbed.  The  condi- 
tions and  the  means,  besides  the  presence  of  carbon-dioxide, 
are  warmth,  water,  light  of  definite  composition,  chloro- 
phyll, and  living  protoplasm.  Warmth  is  necessary  mainly 
as  one  of  the  general  conditions  for  life,  for  at  any  tempera- 
ture at  which  protoplasmic  activity  is  possible  diffusion  and 
chemical  combination  are  also  possible.  Water  is  necessary, 
for  its  component  atoms  become  chemically  combined  with 

*  Brown,  H.  T.  The  fixation  of  carbon  by  plants.  Nature,  Sept.  14. 
1899. 


NUTRITION  51 

the  carbon  atoms  of  the  carbon-dioxide.  Light  rays  of 
definite  wave-lengths  are  the  force  employed  by  the  living- 
protoplasm  to  separate  the  carbon,  hydrogen,  and  oxygen 
atoms  and  to  reunite  them  into  another  and  more  complex 
molecule  than  either  carbon-dioxide  or  water.  Chlorophyll 
is  the  pigment  which  absorbs  these  light  rays.  Living  pro- 
toplasm is  the  only  agent  yet  known  which  can  combine 
these  twro  universally  occurring  and  extremely  stable  com- 
pounds, carbon-dioxide  and  water,  into  food  for  living 
beings. 

CHLOROPHYLL 

Chlorophyll  is  a  pigment,  more  properly  a  mixture  of  two 
or  more  pigments,  of  unknown  chemical  composition,  proba- 
bly nitrogenous,  extremely  unstable,  lifeless,  but  enclosed  in 
the  living  protoplasm,  usually  in  definite  living  organs  of 
the  protoplasm  called  plastids  or  chromatophores.  In  the 
cells  of  the  lowest  algae,  the  Schizophyceae,  the  peripheral 
layers  of  protoplasm  contain  a  mixture  of  blue  and  green 
pigments  which  give  to  them  their  peculiar  blue-green  or 
olive-green  hue.  Whatever  may  be  the  function  of  the  blue 
pigment,  soluble  in  cold  water,  it  is  accompanied  by  a  green 
pigment,  or  a  mixture  of  pigments  which  is  green,  of  similar 
composition  and  the  same  functions  as  chlorophyll.  Higher 
algae  consist  of  cells  more  differentiated  in  that  the  chloro- 
phyll is  confined  to  definite  parts  of  the  peripheral  layers  of 
protoplasm,  in  bodies  possessed  of  denser  structure  than  the 
rest  of  the  protoplasm,  apparently  of  a  living  framework 
between  the  parts  of  which  the  chlorophyll,  as  a  dense  and 
oily  liquid,  is  enclosed.  With  further  division  of  labor  in 
the  cell  the  size  of  the  chromatophores  is  decreased,  their 
numbers  are  increased,  and  the  enclosed  pigments  become 
denser.  By  these  means  the  effectiveness  of  the  chromato- 
phores themselves  is  increased,  their  smaller  size  admits  of 
their  moving  about  in  the  cell,  thus  enabling  them  to  oc- 
cupy different  positions  at  different  times,  but  at  all  times 
the  most  advantageous  ones;  at  the  same  time,  a  smaller 
proportion  of  the  cytoplasm  is  limited  to  the  work  of  food- 
manufacture. 


52  PLANT  PHYSIOLOGY 

The  chief  physical  characteristics  of  chlorophyll,  which  is 
probably  made  as  well  as  contained  in  the  chromatophores, 
are  its  complete  insolubility  in  water,  its  ready  solubility 
in  alcohol,  its  fluorescence,  and  the  absorption  bands  of  its 
spectrum.  By  reason  of  its  insolubility  in  water,  it  is  pos- 
sible to  separate  chlorophyll  from  the  other  pigments  some- 
times masking  its  presence  in  the  cell — the  blue  of  the  Sclri- 
zophycetP,  the  brown  of  the  PhseopliyceiP,  the  red  of  the 
Carpophycete,  and  the  red  and  purple  coloring-matters 
often  present  in  the  cell-sap  of  higher  plants.  *  Having  ex- 
tracted these  pigments  in  cold  water,  alcohol  may  be  used 
to  dissolve  the  chlorophyll,  which  will  certainly  go  into 
solution  more  rapidly,  and  possibly  also  with  less  chemical 
change,  if  the  alcohol  be  applied  hot.  Besides  the  chloro- 
phyll, this  extract  will  contain  other  cell-contents  soluble  in 
alcohol  and  not  removed  by  the  water,  notably  the  fats  and 
oils.  The  alcoholic  solution  is  ordinarily  very  unstable,  for 
though  it  will  retain  its  color  for  a  few  days  if  kept  in 
the  dark,  the  change  in  color,  which  begins  at  once,  continu- 
ing only  less  rapidly  than  in  the  light,  is  evidence  of  chem- 
ical changes  finally  resulting  in  its  complete  destruction. 
From  a  freshly  made  alcoholic  extract  of  chlorophyll  at 
least  two  colored  substances  may  be  separated  in  solution. 
Benzine  or  gasoline  shaken  with  an  equal  volume  of  the 
alcoholic  extract  will  take  up  the  chlorophyll-green  proper, 
leaving  a  yellow  pigment  or  pigments  in  solution  in  the 
alcohol.  This  will  not  secure  a  complete  separation  of  the 
two,  however,  for  some  of  the  yellow  pigment  will  remain  in 
the  benzine.  This  yellow  pigment  is  usually  called  carotin. 
To  separate  chlorophyll-green  in  anything  approaching 
purity  is  a  difficult  task.  The  extreme  instability  of  the  pig- 
ments enclosed  in  the  chromatophores  makes  it  almost  cer- 
tain that  they  will  be  acted  on  by  the  organic  acids  present 
in  the  cells  and  extracted  with  them.  Furthermore,  the 
solvents  of  chlorophyll  (alcohol,  ether,  etc.)  extract  the 
fats  and  oils  also.  Although  many  attempts  have  been 

*  Molisch,  H.  Dae  Phycoerythrin,  seine  Krystallieirbarkeit  und  chem- 
ische  Natur.  Bot.  Zeitung  Bd.  52  1894.  Das  Phycocyan,  ein  krystal- 
lisirbarer  Eiweisskorper.  Ibid.,  Bd.  54,  1895. 


NUTRITION  53 

made  to  isolate  chlorophyll  none  has  yet  been  really  suc- 
cessful. * 

The  fluorescent  quality  of  chlorophyll,  more  evident  in 
its  solutions  than  in  the  leaf,  is  revealed  by  the  blood-red 
color  visible  when  light  is  reflected  from  it.  However,  the 
physiological  significance  of  this  fluorescence  is  not  under- 
stood. 

The  spectrum  of  a  fresh  alcoholic  solution  of  chlorophyll 
is  essentially  the  same  as  that  of  chlorophyll  in  the  unin- 
jured leaf.  The  spectra  vary  in  any  case  with  the  density 
of  the  solution,  the  thickness  of  the  layer  through  which  the 
light  passes,  whether  of  solution  or  of  uninjured  tissue,  and 
also  with  the  plants  examined.  The  different  shades  of 
green  leaves  similarly  exposed  to  light,  the  differences  in 
their  spectra,  and  in  the  spectra  of  solutions  derived  from 
them,  and  the  different  substances  obtained  in  the  chemical 
examination  of  chlorophyll  solutions  from  different  species 
of  plants,  all  point  to  there  being  at  least  different  propor- 
tions of  the  component  pigments  of  the  mixture  called 
chlorophyll,  if  not  different  pigments  in  the  chlorophyll  of 
different  plants.  Though  the  function  of  chlorophyll— the 
absorption  of  energy  in  the  form  of  light  rays — is  the  same 
in  all  green  plants,  it  by  no  means  follows  that  the  chloro- 
phyll of  all  plants  is  equally  efficient  in  absorbing  light  rays 
or  that  exactly  the  same  rays  are  used  in  all  plants.  Slight 
differences  in  the  composition  of  chlorophyll  would  cause 
corresponding  differences  in  the  quality  and  in  the  quantity 
of  the  energy  absorbed  by  it. 

The  spectra  obtained  from  light  which  has  passed  through 
a  leaf,  and  of  alcoholic  solutions  of  chlorophyll,  are  char- 
acterized by  dark  bands  called  " absorption  bands,"  the 
broadest  and  darkest  of  which  are  between  the  lines  B  and 
C,  and  E  and  F,  of  the  normal  solar  spectrum,  that  is,  in 

*  The  literature  of  the  subject  is  voluminous.  References  to  the  older 
literature  may  be  found  in  Monteverde's  papers  ( Acta  Horti  Petropolitani, 
t.  XIII. .  1893)  ;  Kohl's  (Bot.  Centralbl.,  Bd.  73  et  seq.,  1898)  ;  Bode's 
(ibid.,  Bd.  77,  1899),  and  in  an  extended  paper  promised  by  Tswett,  in 
his  preliminary  announcement  in  Bot.  Centralbl.  Bd.  81.  1900,  also  in  a 
collective  review  by  Czapek  in  Bot.  Zeitung.  II.  Abth..  May,  1900. 


54 


PLANT  PHYSIOLOGY 


the  red,  orange,  and  yellow,  and  in  the  green  and  blue  parts 
respectively.  In  the  normal  solar  spectrum  are  certain  dark 
narrow  lines,  named  after  their  discoverer,  Fraunhofer's 
lines.  These  indicate  parts  of  the  spectrum  in  which  there 
is  no  light  or  other  known  form  of  energy.  When  a  chloro- 
phyll screen  is  interposed  in  the  path  of  a  beam  of  sunlight 
the  spectrum  differs  from  that  of  normal  light  in  that  be- 
tween the  Fraunhofer  lines  B  and  C,  and  E  and  F,  energy 
in  the  form  of  light  has  been  absorbed  by  the  chlorophyll. 
This  energy,  absorbed  by  the  lifeless  chlorophyll  pigment,  is 
what  is  employed  by  the  living  chloroplastid  in  elaborating 
a  nutritious  compound  from  carbon-dioxide  and  water.  The 


Blue 


2  Leaves 


B  C  D  E  b 

FIG.  1.     CHLOROPHYLL  SPECTRA.     (AFTER  REINKE.) 

accompanying  figure  indicates  the  absorption,  as  reported 
by  Reinke,*  of  an  alcoholic  extract  of  green  leaves,  and  of 
two  leaves  and  of  seven  leaves  of  Iinpatiens  parvitiorn 
interposed  in  the  path  of  a  beam  of  light. 

The  figure  shows  plainly  that  light  which  has  passed 
through  one  layer  of  chloroplastids  is  essentially  and  obvi- 
ously poorer  in  energy  than  that  which  falls  upon  the  first 
layer.  The  chromatophores  of  a  single-celled  green  plant, 
and  of  plants  consisting  of  filaments  or  single  layered  films 
of  cells,  receive  beams  of  light  containing  the  maximum  of 
energy — assuming  that  none  is  absorbed  by  the  medium  in 
which  they  live.  In  plants  composed  of  masses  of  cells, 
only  the  outermost  layer  of  chlorophyll-containing  cells  re- 
ceives the  maximum  amount  of  light  and  energy.  The 
deeper  layers  evidently  receive  less  and  less.  A  moment's 
thought  will  lead  one  to  conclude  that  only  the  peripheral 
la.yers  of  cells  in  a  massive  plant  or  organ  will  receive  much 

*  Reinke.  J.  Untersuchungen  iiber  die  Einwirkung  des  Lichtes  auf  die 
Sauerstoffausscheidung  der  Pflanzen.  2te  Mittheilung:  Wirkung  der 
einzelnen  Strahlengattungen  des  Sonnenlichtee.  Bot.  Zeitung.  1884. 


NUTRITION  55 

utilizable  light,  and  that  flat  organs  like  leaves  cannot 
profitably  consist  of  many  layers  of  chlorophyll-containing 
cells.  Examination  of  plant  parts  and  organs  shows  that 
chlorophyll  is  developed  in  greatest  amount  in  the  outer- 
most layers  of  the  parts  exposed  to  light,  7.  e.  in  the  outer- 
most of  the  cortical  parenchyma  cells  and  in  the  uppermost 
of  the  mesophyll  layers  of  the  leaf.  Ferns  and  other  shade 
plants  develop  chloroplastids  in  the  epidermal  cells,  but  in 
plants  growing  in  situations  abundantly  lighted,  it  is  not 
necessary  to  dispose  the  chlorophyll  in  the  cells  of  the  ex- 
posed surfaces. 

The  effectiveness  of  chlorophyll  as  an  absorber  of  needed 
energy  is  proved  by  the  excessively  small  quantity  which  is 
adequate  to  the  needs  of  the  plant.  According  to  the  fre- 
quently quoted  estimates  made  by  Tschirch,*  there  are  be- 
tween 0.2  and  1.00  of  a  gram  per  square  meter  of  leaf- 
surface,  or,  in  Ricinus,  about  0.1%  of  the  chloroplastids  is 
chlorophyll  pigment,  and  in  leaves  2-4%  of  the  ash-free  dry- 
substance,  1.75-3.50%  of  the  total  dry-substance,  is  chloro- 
phyll. As  might  be  expected  from  the  small  amount  of 
chlorophyll,  the  amount  of  energy  absorbed  by  the  leaf 
bears  only  a  small  proportion  to  the  total  amount  falling 
upon  it  as  sunshine  on  a  bright  day.  Brownt  estimates 
that  600,000  calories  per  square  meter  fall  on  a  sunflower 
leaf  per  hour  on  a  bright  August  day  in  England — certainly 
more  than  this  in  many  other  parts  of  the  world.  Sun- 
flower leaves  under  these  conditions  form  0.8  gram  of 
carbohydrate  per  square  meter  per  hour,  which  requires 
3,200  calories  or  0.5%  of  the  available  energy.  In  diffuse 
daylight  the  percentage  of  energy  absorbed  and  used  is 
more  in  proportion  to  the  amount  received.  It  would  seem, 
then,  that  the  chlorophyll  apparatus  is  so  adjusted  as  to 
make  the  most  of  the  amount  of  light  daily  available  rather 
than  being  most  efficient  only  when  the  light  is  unusually 
strong. 

The  relative  values,  as  sources  of  energy,  of  the  rays  com- 
posing sunlight  are  indicated  by  the  proportions  of  the 

*  Tschirch.  A.     Angewandte  Pflanzenanatomie,  p.  57  et  seq.,  1889. 

t  Brown,  H.  T.     The  fixation  of  carbon  by  plants.    Nature.  Sept.,  1899. 


66  PLANT  PHYSIOLOGY 

absorption  bands  of  the  chlorophyll  spectrum,  the  largest 
and  darkest  being  in  the  most  luminous  part  of  the  spec- 
trum, the  next  toward  the  actinic  violet  end.  Sachs's 
classical  experiment  of  cultivating  plants  behind  colored 
screens — the  one  consisting  of  a  layer  1.2-1.5  cm.  thick  of  a 
saturated  solution  of  potassic  dichromate,  which  absorbs 
the  blue  and  violet  rays  wholly,  but  allows  the  others  to 
pass;  and  the  other  of  a  layer  1  cm.  thick  of  a  dark 
solution  of  cuprammonia,  which  absorbs  only  the  red, 
orange,  and  yellow  rays— shows  that  the  luminous  rays 
absorbed  are  used  to  elaborate  about  90%  (starch  serving 
as  the  indicator)  of  the  food  first  formed  by  the  plant, 
while  the  more  actinic  rays  yield  only  5-7%.  Thus  the  en- 
ergy absorbed  by  chlorophyll  as  indicated  by  the  two  main 
absorption  bands  in  the  spectrum  is  made  to  produce  95- 
\  97%  of  all  the  carbon  food  compounds  first  formed  in  the 
plant.  The  remaining  5-3%  are  formed  by  the  other  visi- 
ble rays  absorbed.  The  thermal  ultra-red  rays  and  the 
actinic  ultra-violet  rays,  although  absorbed  by  the  plant, 
are  not  used  in  manufacturing  non-nitrogenous  foods. 
.Engelmann's  bacteria  method, f  now  almost  as  famous  as 
Sachs's  colored  screens,  though  applicable  only  to  small 
plants,  affords  much  more  delicate  and  exact  evidence  of 
the  statements  just  made.  The  method  consists  in  the 
employment  of  motile  aerobic  bacteria  in  conjunction  with 
small  green  aquatic  plants.  Beneath  the  stage  of  the 
microscope  a  prism  is  so  placed  as  to  throw  a  short  spec- 
trum upon  the  object  on  the  slide.  Upon  the  slide,  in  a 
drop  of  water  containing  motile  bacteria,  is  laid  a  filament 
of  (Edogonium,  Spirogyra,  or  some  similar  plant.  Around 
those  cells  illuminated  by  the  red,  orange,  and  yellow,  rays, 
the  bacteria  will  congregate  in  greatest  numbers,  there 
moving  with  the  utmost  activity.  Around  other  illumi- 
nated cells  also  the  bacteria  will  collect,  but  in  much  smaller 

*  Sachs,  J.  von.  Wirkungen  farbigen  Lichte  auf  Pflanzen.  Gesammelte 
Abhandhmgen,  Bd.  I.,  p.  261  et  seq. 

t  Engelmann,  Th.  W.  Botanische  Zeitung,  1882,  1883,  1884,  1886, 
1887.  Also  Die  Erscheinungsweise  der  Sauerstoffausscheidung  chlorophyll- 
haltiger  Zellen.  Verhandl.  d.  Amsterdamer  Academic,  1894. 


NUTRITION  57 

numbers  and  in  numbers  about  equal  in  all  the  colors, 
though  somewhat  more  in  the  green-blue. 

That  both  living  protoplasm  and  chlorophyll  pigment  are 
necessary  is  shown,  on  the  one  hand,  by  the  ineffectiveness 
of  plastids  well  developed  but  colorless,  and,  on  the  other, 
by  the  complete  inability  of  chlorophyll  solutions  either 
themselves  to  liberate  oxygen  from  carbon-dioxide  in  the 
light  or  to  enable  colorless  plastids  to  do  so.  The  energy 
needed  must  be  absorbed  by  chlorophyll  in  direct  contact 
with  and  enclosed  in  living  protoplasm,  otherwise  both 
energy  and  chlorophyll  are  useless.  Chlorophyll  grains 
mechanically  isolated  from  living  cells  continue  for  a  time  to 
absorb  carbon-dioxide  and  to  give  off  oxygen,  as  may  be 
shown  by  motile  aerobic  bacteria.*  But  if  such  isolated 
plastids  are  so  treated  that  they  pass  over  into  a  state  of 
anaesthesia,  they  no  longer  absorb  carbon-dioxide,  and 
liberate  oxygen  although  they  still  absorb  light  rays.  The 
living  protoplasmic  body  of  the  plastids,  together  with  the 
chlorophyll  pigment  contained  in  it,  are  the  organs  by 
which  plants  begin  the  manufacture  and  elaboration  of  their 
food.  These  organs  are  to  a  high  degree  independent  of  the 
other  organs  of  the  cell  and  of  the  cytoplasm  itself,  but  not 
entirely  so;  they  are  still  cell-organs. 

For  the  formation  of  chlorophyll  in  cells  which  normally 
contain  it  two  conditions  may  be  mentioned  as  of  special 
importance,  namely,  illumination  and  the  presence  of  iron. 
Most  plants  fail  to  produce  chlorophyll  in  darkness,  al- 
though a  larger  number  than  was  formerly  supposed  are 
now  known  to  produce  it  regardless  of  the  illumination. 
Among  these  may  be  mentioned  the  seedlings  of  conifers 
and  maples.  The  influence  of  light  on  its  formation  in  most 
plants  must  be  regarded  as  a  stimulus  rather  than  a  pre- 
requisite, although  the  work  of  the  chlorophyll  pigment  can 
be  done  only  in  the  light.  Then  only  is  there  energy  to  be 
absorbed,  and  the  radiant  energy,  falling  upon  the  cell,  will 
stimulate  it  to  develop  a  more  effective  absorbent  than  wall 
and  cytoplasm  afford.  The  action  of  iron  also,  since  it  is  in 

*  Ewart,  A.  J.  Can  isolated  chloroplastids  continue  to  assimilate? 
Bot.  Centralblatt,  Bd.  75.  No.  2,  1898,  and  other  papers  there  cited. 


58  PLANT  PHYSIOLOGY 

all  probability  not  a  constituent  element  of  the  chlorophyll 
pigments,  must  be  either  that  of  a  stimulant  to  the  living 
protoplasm  to  form  chlorophyll,  or  else  in  some  purely  non- 
vital  way  an  assistant  in  the  synthesis  of  these  color-com- 
pounds. The  former  supposition  seems  more  probable,  but 
neither  has  any  strong  evidence  in  its  favor.  However, 
neither  light  nor  iron  will  bring  about  the  production  of 
chlorophyll  in  cells  which  do  not  contain  chromatophores  as 
living  organs.  The  cells  of  animals,  fungi,  and  certain  of  the 
phanerogamic  parasites  and  saprophytes,  contain  no  chro- 
matophores, no  organs,  therefore,  capable  of  forming,  con- 
taining, and  using  the  chlorophyll  pigments. 

PHOTOSYNTHESIS 

The  absorption  of  carbon-dioxide  and  the  combination  of 
water  with  it  by  green  plants  have  for  years  been  mislead- 
ingly  termed  assimilation  or  carbon-assimilation.  The  word 
assimilation  may  much  more  correctly  and  significantly  be 
applied  to  the  working  up,  by  the  living  organism,  of  sub- 
stances already  somewhat  resembling  in  constitution  the 
substance  of  the  organism  itself.  The  word  really  means 
making  like.  No  organism  can  make  carbon-dioxide  and 
water  like  itself  till  it  has  combined  them.  The  food  formed 
by  combining  the  two  innutritions  substances,  carbon-diox- 
ide and  water,  can  be  used  directly  only  as  fuel,  for  respira- 
tion. It  must  be  modified,  added  to,  elaborated,  changed 
in  various  ways  not  yet  understood,  until  it  becomes  chem- 
ically and  physically  like  the  component  matters  of  the 
body  of  the  organism.  Combining  carbon-dioxide  and  wa- 
ter is  food-manufacture — a  synthetic  process;  the  modifica- 
tion of  this  food  which  makes  it  immediately  available  as 
constructive  material  is  assimilation — not  necessarily  a  syn- 
thetic process.  Since,  as  we  have  seen,  the  energy  by  which 
the  synthetic  process  is  accomplished  comes  from  the  sun, 
the  name  photosynthesis  has  been  proposed  for  it. 

Besides  the  substances  absorbed— carbon-dioxide  and 
water — and  the  final  products  of  the  synthesis — starch  or 
some  equivalent — nothing  is  known  of  the  chemistry  of  pho- 
tosynthesis. Only  the  relative  numbers  of  the  atoms  form- 


NUTRITION 


59 


ing  the  starch  molecule  are  known,  and  therefore  it  is  custom- 
ary to  give  either  the  minimum  formula  or  to  indicate  by 
n  that  the  starch  molecule  should  be  represented  by  some  mul- 
tiple, probably  high,  though  still  undetermined,  of  the  mini- 
mum proportional  formula.  The  proportional  amounts  of 
the  end  substances  are  indicated,  therefore,  by  this  equation 

n(6  CO, +  5  H90)=n(C6H1.Ofc  +  6  Of) 

Starch,  though  frequently  termed  the  first  visible  product  of 
photosynthesis,  is  not  formed  in  all  chlorophyll-containing 
cells  (e.g.  those  of  onion  leaves),  and  should  never  be  re- 
garded as  the  first  product  of  photosynthesis.  The  com- 
plexity of  the  starch  molecule  and  the  simplicity  of  the 
molecules  of  carbon-dioxide  and  water,  indicated  by  their 
respective  proportional  formulae  and  still  more  plainly  by 
their  structural  formulae,  *  make  it  extremely  doubtful  if  even 
the  commoner  sugars  are  formed  directly.  These,  because 
they  are  formed  and  remain  in  solution,  can  be  recognized 
only  by  chemical  means,  while  starch,  a  solid  with  a  refrac- 
tive index  different  from  the  other  substances  in  the  cell,  is 


*  The  structural  formula  of  CO2  is  probably  O  =  C 
"  "  "        "  H2O  "        H  — O  — H 

"        "  n  (C.  H10  0§)  may  be 


As  examination  of  this  structural  formula  shows, 
CfiH10O5  is  only  an  approximate  proportional  for- 
mula for  starch.  It  would  be  more  correct  to  rep- 
resent starch  thus,  n  (  C6  H12  O6 )  —  ( n  —  1 )  H2O. 
Evidently,  the  higher  the  multiple  represented  by  n, 
the  nearer  this  formula  will  come  to  the  propor- 
tional C6  H10  O5.  These  proportions  were  obtained 
by  analysis.  The  exact  numbers  can  be  learned  only 
by  synthesis,  and  no  one  has  yet  been  able  to  make 
starch. 


=  O 


n 


H 

I 
H-C-OH 

H—  C-OH 

H-C-OH 

I 
H-C-OH 

I 
H-C-OH 


H—  C-OH 
H-C-OH 

H-C-OH 

I 
H-C-OH 

H—  C=  O 


60  PLANT  PHYSIOLOGY 

visible  as  soon  as  formed,  and  it  can  be  identified  by  the 
familiar  blue-black  color  imparted  to  it  by  iodine. 

In  a  very  considerable  number  of  plants  and  plant  organs, 
starch  is  never  formed,  whether  as  the  immediate  product 
of  photosynthesis  or  as  food  in  store  for  future  use.  In 
these  the  food  remains  as  sugars  in  solution,  accumulates  in 
visible  form  as  drops  of  oil,  or  may  be  stored  as  reserve 
cellulose.  It  is  in  general  true  that  in  plants  and  in  organs 
which,  for  any  reason,  should  be  especially  light,  oil  sepa- 
rates instead  of  starch  :  e.  g\  in  the  Diatoms,  most  of  which 
float  either  free  or  attached ;  in  many  of  the  Phaeophyceae, 
in  the  Characese  (except  the  bulbils),  and  in  such  light 
and  mechanically  weak  structures  as  the  cylindrical  leaves 
of  onion,  etc.  The  specific  gravity  of  starch  is  higher,  of  oil 
lower,  than  that  of  water.  Their  nutritive  values  may  be, 
probably  are,  about  equal.  Their  values  as  sources  of  en- 
ergy through  respiration  are  not  equal.*  The  advantage 
of  oil  as  reserve  food,  or  as  the  immediate  product  of  pho- 
tosynthesis, where  lightness  is  important,  is  evident.  Sugar 
never  crystallizes  in  the  cell,  it  remains  always  in  solution, 
osmotically  active  (though  comparatively  weak),  readily 
diffusible,  and  although  occurring  much  more  commonly  as 
food  immediately  available  or  in  transit  from  one  part  of 
the  plant  to  another,  it  is  sometimes  the  form  in  which 
food  is  accumulated  and  stored. 

From  considerations  insufficiently    supported  by  experi- 

*  Complete  oxidation  (perfect  respiration)  of  oil  yields  half  again  as 
many  calories  per  gram-molecule  as  starch  and  sugar.  See  Chemiker 
Kalender  where  heat  of  combustion  of  vegetable  oils  is  given,  about  932 
Gals  while  that  of  starch  and  sugar  is  about  709  Cals. 

According  to  Stahl  (Jahrb.  f.  wiss.  Botanik,  Bd.  34,  p.  565,  1900), 
constantly  submersed  Chlorophycea?,  which  run  no  risk  of  drying  up 
during  a  period  of  vegetation,  form  starch,  while  on  the  other  hand,  those 
alg*  subjected  to  frequent  drying  up  form  sugar.  Water  would  evaporate 
less  and  less  rapidly  from  a  sugary  or  other  solution  of  increasing  density. 
The  rate  of  evaporation  would  not  be  affected  in  any  way  by  starch  or 
other  substances  not  in  solution.  Vaucheria  and  certain  other  algae, 
some  species  of  which  are  subject  to  drying  up,  contain  oil  in  quantity. 
This  cannot  reduce  the  loss  of  water  by  evaporation.  Are  these  algse 
therefore  less  well  protected  against  harm  by  drying,  or  is  Stahl's  con- 
jecture a  mistaken  one? 


NUTRITION  61 

ment  upon  the  living  plant,  though  based  upon  the  relations 
of  the  members  of  the  sugar  group  to  one  another,  a  series 
of  equations  suggesting  the  possible  (some  say  probable) 
stages  of  the  photosynthetic  process  has  been  proposed  by 
organic  chemists.  *  The  following,  proposed  by  von  Baeyer, 
has  much  in  its  favor : 

CO,  +  H2O  =  HCOOH  +  O  =  HCOH  +  O2 

( Formic  Acid )  ( Formic  Aldehyde ) 

6n  (HCOH)  ==  n  (C6H,O6)  ==  n(C6H10O5  +  H2O) 

(Sugar)  (Starch) 

To  this  series  of  reactions  there  are  evident  objections :  e.  g. 
formic  acid  and  formic  aldehyde  are  extremely  poisonous, 
they  cannot  be  formed  free  in  the  cell  unless  their  conversion 
into  harmless  substances  is  Jnstantaneous.  The  union  of 
any  of  the  alkaline  elements  with  formic  acid  would  imply 
the  decomposition  of  the  salts  in  which  these  elements  enter 
the  cell,  namely,  the  phosphates,  nitrates,  sulphates,  and 
chlorides.  The  decomposition  of  these  salts,  assuming  that 
it  could  be  immediately  accomplished  by  formic  acid  or  other- 
wise, would  set  free  in  the  cell  acids  which,  though  less  pois- 
onous than  formic  acid,  would  still  be  injurious.  To  neu- 
tralize these  another  series  of  reactions  would  be  necessary. 
Pursuing  this  theoretical  discussion  further  along  this  line  is 
unnecessary,  for  after  all,  assuming  that  the  formic  acid  is 
neutralized  by  sodium,  potassium,  or  any  other  element  in- 
variably present  in  combination  in  the  cell,  and  that  the  re- 
duction of  this  to  an  aldehyde  takes  place,  the  alkaline  element 
must  be  eliminated  to  permit  the  condensation  (polymeriza- 
tion )  of  formic  aldehyde  to  sugar,  and  the  element  must  be 
united  again  with  some  inorganic  acid  to  form  a  harmless 
salt.  There  must,  therefore,  be  at  least  one  other  series  of 
reactions  taking  place  simultaneously  with  the  others. 

Loew  and  Bokornyf  have  conducted  experiments  on  Spiro- 
gyra,  using  as  sole  source  of  carbon  a  0.1%  solution  of 

*  Baeyer,  A.  von.  Uber  die  Wasserentziehung  und  ihre  Bedeutung  fur 
das  Pflanzenleben  und  die  Gahrung.  Berichte  d.  Deutch.  Chemischen 
Gesellsch.,  Bd.  III.,  1870.  Fischer,  E.  Synthesen  in  der  Zuckergruppe  I. 
ibid,  Bd.  XXIII.,  1890.  II.  ibid.,  Bd.  XXVII.,  1894. 

t  Loew,  O.  Ernahrung  von  Pflanzenzellen  mit  Formaldehyd.  Botan. 
Centralblatt,  Bd.  XLIV.,  p.  315-f .  1890.  Bokorny,  Th.  tJber  Starke- 


62  PLANT  PHYSIOLOGY 

oxymethyl-sodium-sulphonate,  which  can  readily  be  broken 
up  at  a  regular  rate  into  formic  aldehyde  and  acid  sodic 
sulphite.  Spirogyra  thrives  on  this  diet,  but  this  fact  does 
not  justify  the  conclusion  of  the  experimenters  that  formic 
aldehyde  is  first  formed  or  formed  at  all  by  green  plants 
in  the  photosynthesis  of  food;  it  simply  proves  that  Spi- 
rogyrn  is  able  under  the  conditions  of  the  experiment 
to  supply  itself  with  carbon  from  other  than  the  usual 
source. 

Emil  Fischer's  hypothesis  regarding  the  formation  of 
sugar  ( starch,  etc. )  by  green  plants,  *  represents  the  mature 
ideas  of  an  eminent  chemist;  it  is  supported  by  no  experi- 
mental evidence  from  physiologists,  yet  it  is  not  out  of 
place  here.  "  The  formation  of  sugar  is  accomplished,  accord- 
ing to  the  plant-physiologists,  in  the  chlorophyll  grain,  itself 
composed  of  optically  active  substances  exclusively.  I  be- 
lieve that  the  formation  of  a  compound  of  carbon-dioxide  or 
formic  aldehyde  with  these  precedes  the  formation  of  sugar, 
and  that  then  the  condensation  to  sugar,  because  of  the 
asymmetry  of  the  whole  molecule,  is  accomplished  asymmet- 
rically. The  elaborated  sugar  is  torn  away  from  the  whole 
(chlorophyll)  molecule  and  then  used  by  the  plant  for  the 
preparation  of  its  other  component  organic  compounds." 

The  elaboration  of  carbohydrates  by  the  more  or  less 
indirect  methods  successfully  followed  by  chemists,  furnishes 
little  more  than  plausible  hypotheses  as  to  how  the  green 
plant  elaborates  them.  The  chlorophyll  apparatus  is  not 
identical  in  structure,  probably  much  less  in  its  operation, 
in  all  green  plants,  and  it  is  not  reasonable  to  suppose  that 
the  different  substances  formed  by  its  operation  are  all 
elaborated  in  the  same  ways.  Formic  aldehyde  may  be  an 
intermediate  product  in  the  formation  of  sugars  and  starch, 
and  it  may  not ;  it  is  hard  to  see  how  it  can  be  in  the  for- 
mation of  fats  and  oils.  Aided  by  the  energy  absorbed  by 
the  lifeless  chlorophyll  pigments,  the  living  chloroplastids 
may  form  definite  new  molecules  by  uniting  carbon-dioxide 

bildung  aus  Formaldehyd.    Ber.  d.   Deutch.    Bot.  Gesellsch.,  Bd.  IX.,  p. 
103+,  1891,  and  Biologisches  Centralblatt,  Bd.  XVII.,  1897. 
*  Fischer,  Emil.    Synthesen  in  der  Zuckergruppe,  II.  1.  c.,  p.  8231. 


NUTRITION  63 

or  for  mic  aldehyde  with  the  already  complex  molecules  of  the 
pigments.  By  the  same  living  agent,  with  or  without  solar 
energy,  these  molecules  may  be  split  into  chlorophyll  and 
sugar,  but  the  chlorophyll  pigments  are  so  many  that  no 
one  reaction  or  series  of  reactions  will  truthfully  represent 
the  process  in  all  plants.  The  problem  remains  unsolved, 
and  suggestive  as  the  deductions  of  chemists  are,  its  solu- 
tion is  to  be  expected  from  physiologists  equipped  with 
chemical  knowledge  rather  than  from  chemists  devoid  of 
physiological  experience.  ^ 

In  summing  up  this  part  of  our  study  of  the  nutrition  of 
plants,  we  may  say  that  the  non-nitrogenous  foods,  which 
first  appear  in  the  cells  as  sugars  (cane  and  grape),  mannit, 
etc.,  starch,  fats,  and  oils,  are  formed  from  carbon-dioxide 
and  water  in  the  chromatophores  (starch)  or  in  the  next 
adjacent  cytoplasm  (oil)  by  these  living  organs  of  the  cell, 
the  energy  necessary  for  this  synthetic  process  being  ab- 
sorbed by  the  lifeless  chlorophyll  pigments  and  used  by  the 
living  protoplasm. 

These  non-nitrogenous  foods  accumulate  temporarily  in 
the  plastids  ( starch ) ,  the  cytoplasm  ( oil ) ,  and  the  cell-sap 
(sugars).  The  manufacture  of  these  foods  can  be  accom- 
plished only  during  the  hours  when  the  plant  can  secure  the 
necessary  energy,  7.  e.  while  it  is  suitably  illuminated.  The 
removal  of  the  manufactured  product  goes  on  independently 
of  the  illumination,  for  whenever  the  food  is  in  soluble  form 
and  in  solution,  it  will  diffuse  out  of  the  cells  in  which  it  is 
made  into  others  containing  smaller  amounts  of  these  sub- 
stances. This  diffusion  goes  on  constantly,  the  rate  of 
diffusion  varying  only  with  the  differences  in  the  amounts  of 
the  foods  in  the  different  cells.  While  the  plant  is  strongly 
illuminated,  however,  the  rate  of  manufacture  is  higher  than 
the  rate  of  removal  by  diffusion,  and  the  food  accumulates 
at  this  time  in  the  cells  forming  it.  Hence,  in  the  early 
morning,  we  may  find  the  chlorophyll-containing  cells  free 
from  starch,  oil,  and  anything  more  than  small  amounts  of 
sugar,  at  the  close  of  the  day,  full  of  them.  If,  because  of 
prolonged  or  too  intense  illumination  or  because  of  any 
other  unusual  circumstance,  the  rate  of  manufacture  exceeds 


64  PLANT  PHYSIOLOGY 

the  rate  of  removal  in  twenty-four  hours,  there  will  be  an 
increasing  excess  of  food  in  the  chlorophyll-containing  tis- 
sues. If  this  continues  for  a  number  of  days,  the  leaves 
will  become  filled  with  accumulated  starch,  etc.,  will  become 
abnormally  heavy,  and  by  their  unusual  plumpness,  differ- 
ence in  color,  etc.,  will  give  evidence  of  their  morbid  condi- 
tion. Such  "fatness"  of  leaves  is  sometimes  found  in  nature, 
more  frequently,  however,  in  cultivation.  The  cure  is  easy: 
shading  the  plants  will  reduce  the  rate  of  food  manufacture 
and  permit  the  nightly  emptying  of  the  daily  filled  cells. 

When  food  is  deposited  in  indifmsible  form  in  any  cell,  it 
must  be  dissolved  before  it  can  be  removed.  Water  is  in- 
variably the  vehicle  of  food  substances  in  the  plant  as  in  the 
animal  body,  and  the  foods  are  carried  from  cell  to  cell  only 
in  solution.  Starch  grains,  deposited  either  in  the  chloro- 
plastids,  in  which  the  carbohydrate  is  formed,  or  in  the 
adjacent  cytoplasm,  must  be  converted  into  sugar  and 
dissolved  before  they  can  be  removed.  In  the  chloro- 
phyll-containing cells  which  manufacture  food  in  the  light 
and  accumulate  the  manufactured  product  in  the  form  of 
starch,  diastase  or  some  similar  starch-converting  enzym 
must  be  present  in  the  green  tissues  of  leaves,  and  in  other 
chlorophyll-containing  parts.  To  demonstrate  its  presence, 
however  evident  its  effects  may  be,  is  difficult,  because  of  the 
very  small  amount  required  to  do  the  work  in  the  time 
allowed  the  plant.  To  convert  a  large  amount  of  starch 
quickly  into  sugar,  a  comparatively  large  amount  of  dias- 
tase is  needed.  If  the  time  be  longer,  less  diastase  will  do 
the  same  work.  Its  converting  power  is  astonishingly  great 
— 10,000  times  its  own  weight  of  starch.*  The  removal  of 
food  goes  on  constantly.  Its  manufacture  is  only  periodic. 
A  small  amount  of  diastase  secreted  by  the  cell  will  therefore 
accomplish,  in  the  twenty-four  hours,  the  removal  of  all  the 
starch  normally  deposited  in  the  cell  as  the  result  of  photosyn- 
thetic  activity.  That  diastase  is  formed  in  small  quantities 
in  green  leaves  is  indicated  by  the  investigations  of  Vines.  \ 

*  Schleichert,  F.    Das  diastatische  Ferment  der  Pflanzen.    Halle,   1893. 
t  Vines,  S.  H.    On  the  presence  of  a  diastatic  ferment  in  green    leaves, 
s Annals  of  Botany,  V.,  1891. 


NUTRITION 


65 


The  products  of  photosynthetic  activity  are,  as  we  have 
seen,  food,  water,  and  oxygen.  These,  in  obedience  to  the 
laws  of  diffusion,  pass  out  of  the  cells  in  which  they  are 
formed  into  others  which  contain  less,  or  into  the  air.  The 
oxygen  set  free  may  be  used  in  part  in  respiration,  but  only 
a  small  part  of  the  considerable  quantity  of  oxygen  liber- 
ated in  active  photosynthesis  can  be  respired  during  that 
time  by  the  plant  producing  it.  The  volume  of  oxygen 
given  off  during  the  life-time  of  a  plant  will  equal  that  of 
the  carbon-dioxide  absorbed,  for  respiration  and  photo- 
synthesis balance  each  other.  As  the  volume  of  carbon- 
dioxide  given  off  may  be  used  as  a  quantitative  index  of  the 
respiratory  activity,  so  the  volume  of  oxygen  may  be  used 
as  a  quantitative  index  of  the  photosynthetic  activity ;  but 
for  reasons  corresponding  to  those  discussed  under  Respira- 
tion ( pp.  38, 39 ) ,  these  indices  are  not  exact.  Both  gases  are 
at  the  same  time  concerned  in  other  processes  besides  those 
which  we  have  so  far  separately  examined.  The  accompany- 
ing figure  shows  the  relative  activity  of  photosynthesis  and 
respiration  as  indicated  by  the  volumes  of  CO,  concerned. 


2.3°         7.5°      11.3°     15.8°        20.6° 


29.3°    33°       37.3°     11.7°    46.4°    °C. 


Figure  2.— Photosynthesis  and  respiration  of  a  leafy  branch  of  Rubus 
fruticosus  (after  Kreusler*).  During  photosynthesis  the  branch  was  in 
air  containing  0.3%  CO2,  and  was  illuminated  with  electric  light  approxi- 
mately equal  to  the  diffuse  sunlight.  The  upper  curve  indicates  the  amount 
of  C02  actually  consumed  in  photosynthesis. 

C02  used  in  photosynthesis   I    per  hour  in  each  sq.  dm.  of 

CO2  produced  in  respiration  f  leaf  surface. 

*  Copied  by  Pfeffer,  Pflanzenphysiologie,  Bd.  I.,  p.  321.    Eng.  transl.  I.,  p. 
337,  from  Kreusler's  paper  in  Landwirthschaftliches  Jahrbuch,  Bd.  16, 1887. 
5 


66  PLANT  PHYSIOLOGY 

The  water  liberated  in  the  cell  by  the  condensation  of 
starch  from  sugar  passes  off  in  the  ways  to  be  discussed 
later  in  connection  with  the  transfer  of  water  (see 
p.  137). 

The  food  formed  is  used  by  the  cell  which  forms  it  and  by 
the  other  cells  of  the  plant,  either  immediately  or  later. 
As  we  have  seen,  the  food  accumulates  temporarily,  hence 
only  small  amounts  are  used  immediately.  What  is  removed 
is  soon  disposed  of  in  one  or  more  ways;  fii*st,  it  may  be 
used  at  once  to  build  cell-walls,  or  to  furnish  energy  by  be- 
ing respired ;  second,  it  may  be  stored  as  reserve  food  in  or- 
gans other  than  those  that  produced  it ;  third,  it  may  be  still 
further  elaborated,  serving  as  the  basis  for  the  construction 
of  nitrogenous  food.  This  leads  us  from  the  relations  of 
carbon  in  the  nutrition  of  plants  to  those  of  nitrogen. 

NITROGEN 

Although  as  essential  a  constituent  element  of  the  living 
matter  and  therefore  of  the  food  of  all  organisms  as  car- 
bon, nitrogen  forms  a  much  smaller  percentage  of  the  dry 
weight  of  plants,  and  its  distribution  in  the  body  is  much 
less  uniform.  Carbon  is  found  in  all  the  skeletal  parts  of 
the  body  of  a  plant  as  well  as  in  the  living  matter  of  the 
cells.  The  cell-walls,  containing  no  nitrogen,  composed 
mainly  of  a  cellulose  or  of  some  derivation  of  a  cellulose, 
form  the  skeleton  of  the  whole  plant-body,  of  the  dead  as 
well  as  the  living  parts.  On  the  other  hand,  nitrogenous  com- 
pounds occur  only  in  the  living  parts  or  as  small  remnants 
in  parts  formerly  living.  They  are  found  as  reserve  foods  in 
the  cells  and  as  the  living  constituents  of  the  protoplasm. 
The  amounts  and  the  distribution  of  nitrogen  in  the  plant- 
body  are  indicated  by  the  following  table :  * 

In  mushrooms        7.26%  N.  in  the  dry  weight 

"   lupin  seeds        5.0%     "     "    "       "        " 

"   pea         "  3.5%     "     "    "       "        " 

"   rye  fruits  1.9%     "      "    "       " 

"   green  leaves      3-5  %  "     "    "      " 

11   potato  tubers  0.3  %  "     "  fresh  condition. 

*  Frank,  A.  B.    Lehrbuch  der  Botanik,  Bd.  I.,  p.  563,  1892. 


NUTRITION  67 

These  figures  show  plainly  tnat  where  there  is  the  greatest 
proportion  of  living  cells  there  is  also  the  greatest  amount 
of  nitrogen.  Where  living  cells  are  most  active,  as  in  green 
leaves  and  at  growing  points,  and  where  there  is  the  great- 
est and  most  compact  accumulation  of  stored  food,  as  hi 
some  seeds,  the  larger  part  of  the  nitrogenous  substance 
will  be  found.  In  many  seeds,  and  in  such  places  of  stor- 
age as  potato  tubers,  reserve  food  is  accumulated  mainly  as 
starch,  to  be  worked  up  into  nitrogenous  compounds  only 
as  needed. 

Nitrogen  occurs  in  nature  in  the  uncombined  state,  form- 
ing nearly  four-fifths  of  the  earth's  atmosphere,  and  also 
united  into  compounds  of  very  different  degrees  of  complex- 
ity, solubility,  and  availability.  Some  of  the  compounds 
are  the  results  of  the  constructive  activities  of  living  organ- 
isms (see  p.  71),  others  are  the  products  of  their  waste 
or  decay  (see  p.  36),  and  still  others  originate  indepen- 
dently of  living  organisms.  Some  of  the  nitrogen  com- 
pounds absorbed  by  plants  contain  carbon,  others  do  not. 

The  forms  in  which  nitrogen  is  obtained  by  the  plant  are, 
therefore,  much  more  varied  than  are  the  sources  of  carbon. 
The  supply  of  nitrogen  compounds  is  also  far  less  constant. 
At  times  plants  may  obtain  from  the  air,  as  well  as  from 
the  soil  and  from  the  water,  simple  compounds  of  nitrogen, 
such  as  ammonia  and  other  gaseous  substances,  formed  in 
the  air  only  under  electrical  influences,  or  escaping  into  the 
air  from  terrestrial  sources. 

With  the  exception  of  certain  bacteria,  either  free-living  or 
associated  with  higher  plants,  no  living  organisms  can  use 
the  free  nitrogen  so  abundantly  supplied  to  them  in  the  air. 
They  are  absolutely  dependent  upon  compounds  of  nitrogen. 
Animals  obtain  all  their  nitrogenous  as  well  as  non-nitro- 
genous foods  from  plants;  most  plants  must  obtain  their 
nitrogenous  food-materials  in  the  form  of  nitrates.  In  the 
soil  and  in  fresh  and  salt  water  there  occur,  besides  vari- 
ous nitrates,  other  soluble  nitrogen  compounds,  some  en- 
tirely free  from  carbon,  others  united  with  it  in  more  or  less 
complex  forms.  Substances  of  the  latter  sort  are  the  con- 
stituents or  the  derivatives  of  animal  excreta  or  of  the  dead 


68  PLANT  PHYSIOLOGY 

bodies  of  animals  and  plants.  These  serve  as  the  foods  or 
as  the  food-materials  of  plants  of  various  degrees  of  depen- 
dence, e.  g.  certain  species  of  orchids,  the  so-called  sapro- 
phytes, etc.  Plants  of  this  sort  we  shall  consider  later 
(pp.  78-92). 

The  main  source  of  nitrogen  for  independent  plants,  that 
is,  for  plants  which  contain  chlorophyll  and  which  by  means 
of  it  elaborate  their  own  non-nitrogenous  foods,  is  the  salts 
of  nitric  acid,  which  are  found  in  the  soil  and  in  water. 
The  origin  of  these  compounds  in  the  soil,  once  supposed  to 
be  purely  chemical,  has  only  recently  been  shown,  by  the 
brilliant  researches  of  Winogradsky,  *  to  be  the  result  of 
the  activity  of  micro-organisms  universally  present  in  soil 
These  organisms,  bacteria  of  only  a  few  species  and  of  ex- 
ceedingly small  size,  have  been  isolated  and  cultivated  by 
Winogradsky.  He  found  them  to  be  of  two  sorts,  strikingly 
different  in  their  physiological  activity.  The  one  sort — to 
which  Winogradsky  gave  the  generic  names  Nitrosococcus 
and  Nitrosompnas — oxidize  ammonia  compounds  (e.  g. 
ammonic  sulphate  (NH4)2S04)  to  nitrites,  salts  of  nitrous 
acid.  The  other  sort — belonging  to  a  single  genus,  called  by 
Winogradsky,  Nitrobacter — oxidize  these  to  nitrates,  salts  of 
nitric  acid.  In  the  soil  there  is  no  accumulation  of  nitrous 
acid  or  its  salts,  for  they  are  as  rapidly  oxidized  by  Nitro- 
bacter  as  they  are  formed  by  Nitrosococcus  and  Nitrosonio- 
nas.  Furthermore,  there  accumulates  no  free  nitric  acid,  for 
this  is  neutralized  by  the  carbonates  commonly  present. 
These  reactions  may  be  represented  in  simplest  form  thus  : 
(NH4)0S04  +  3  02  =  2  HN00  +  H2SO4  +  2  H2O 

2  HNO2  +  02  =  2  HN03 
2  HNO3  +  K2C03  =  2  KN03  +  C02  +  H2O 

As  already  suggested  (see  p.  20),  these  nitrogen  bacte- 
ria are  strikingly  different  from  all  other  known  organ- 
isms. Though  they  obtain  the  carbon  needed  for  food- 
manufacture  directly  from  the  air  as  carbon-dioxide,  they 
elaborate  this  without  chlorophyll  or  bacteriopurpurin  ( see 

*  Winogradsky,  S.  Recherches  sur  les  organismes  de  la  nitrification. 
Annales  de  1'Institut  Pasteur,  IV.,  V.,  1889-91.  Zur  Mikrobiologie  des 
Nitrifikationsprozessee.  Centralblatt  f.  Bakteriologie,  2te  Abth.,  II.,  1897. 


NUTRITION  69 

p.  43)  and  without  the  aid  of  light,  obtaining  the  requi- 
site energy  by  the  oxidations  which  they  accomplish  as  aero- 
bic organisms.  *  Unlike  the  purple-bacteria,  the  nitrogen  bac- 
teria play  an  obviously  important  part  in  nature,  preparing 
from  otherwise  useless  materials  those  compounds  of  nitro- 
gen upon  which  the  existence  of  all  other  organisms  depends. 
Like  all  other  substances  concerned  in  the  nutrition  of 
k  organisms,  the  nitrates  are  soluble  and  enter  the  body  only 
in  solution  in  water.  The  nitrates,  especially  potassic  ni- 
trate, which  seems  to  be  the  most  useful  of  these  compounds, 
absorbed  by  the  roots  of  land  plants  and  through  other 
organs  of  floating  aquatic  plants,  are  transferred  by  diffu- 
sion throughout  the  body.  The  elaboration  of  these  into 
organic  compounds  can  probably  be  accomplished  by  all 
living  plant-cells,  but  in  higher  plants,  in  which  division  of 
labor  is  carried  to  a  high  degree,  it  is  accomplished  by 
some  cells  and  not  by  all,  very  likely  in  the  leaves  and 
other  green  parts  or  in  parts  not  far  distant.  The  manu- 
facture of  nitrogenous  foods  is  accomplished  both  in  dark- 
ness and  in  the  deeper  tissues,  and  also  in  the  light  and  in 
superficial  tissues,  f  It  is  probable  that  light,  and  even 
certain  rays  of  light,  favor  the  elaboration  of  nitrogenous 
foods.];  The  action  of  light  is  presumably  that  of  a  stimu- 

*  Godlewski,  E.  Tiber  die  Nitrification  des  Ammoniaks  und  die  Kohlen- 
stoffquellen  bei  der  Ernahrung  der  nitrificierenden  Fermente.  (Polish) 
Reviewed  in  Centralblatt  f.  Bakteriologie,  2te  Abth.,  II.,  p.  458.  Well  sum- 
marized in  Fischer's  Vorlesungen  iiber  Bakterien,  p.  103,  Eng.  trans,  p.  106. 

t  Susuki,  U.  Uber  die  Assimilation  der  Nitrate  in  Dunkelheit.  Botan. 
Centralblatt,  Bd.  75,  p.  289,  1898.  On  the  formation  of  proteids  and 
the  assimilation  of  nitrates  by  phaenogams  in  the  absence  of  light.  Bul- 
letin VHI.,  No.  5,  Imperial  University,  College  of  Agriculture,  Tokyo,  1898. 

+  Godlewski,  E.  Zur  Kenntniss  der  Eiweissbildung  aus  Nitraten  in  der 
Pflanze.  Anzeiger  der  Akademie  d.  Wissenschaften,  Krakau,  March,  1897. 
Laurent,  E.  Recherches  experimentales  sur  Tassimilation  de  1'azote  am- 
moniacal  et  de  1'azotte  nitrique  par  les  plantes  superieures.  Bulletin  de 
1' Academic  Royale  de  Belgique,  t.  32,  1896.  Palladine,  W.  Influence  de 
la  lumiere  sur  la  formation  des  matieres  proteiques  actives  et  sur  Penergie 
de  la  respiration  des  parties  vertes  des  vegetaux.  Rev.  gen.  de  Bot.,  t.  XI., 
1899.  Jost,  L.  Die  Stickstoffassimilation  der  grunen  Pflanzen.  Biol. 
Centralblatt,  Bd.  20,  1900.  (Review  of  subject,  and  a  bibliography, 
to  date.) 


70  PLAN1   PHYSIOLOGY 

lus;  light  is  probably  not  the  direct  source  of  the  energy 
used.  The  energy  used  may  be  wholly  of  chemical  origin, 
the  elaboration  of  nitrogenous  foods  taking  place  by  purely 
ehemosynthetic  processes;  or  it  may  be  furnished  by  the 
physiological  oxidation  (respiration)  taking  place  in  the 
cells  in  which  the  compounds  are  being  formed. 

The  nitrogenous  foods  are  found  in  plants  either  in  soluble 
or  insoluble  form.     The  soluble  compounds  are  the  only  ones 
immediately  useful.    They  can  be  transferred  from  cell  to 
cell,  they  can  be  directly  acted  upon  by  the  various  chemical 
influences  brought  to  bear  upon  them  by  the  living  cells, 
they  can  be   assimilated — converted    into    substances  like 
those  composing  the  living  substance— and  when  so  elabo- 
rated and  assimilated,   they  become  a  part  of  the  living 
protoplasm.     In  soluble  form  they  occur  mainly  as  amides, 
of  which  asparagin  is  the  most  abundant  and  probably  the 
most  important.     The  amides  are  found  in  greatest  abun- 
dance in  parts  where  the  formation  and  growth    of   new 
cells    are   most    rapidly    taking   place,    that   is,    in    those 
parts  which   demand   most  food  of   non-nitrogenous    and 
nitrogenous  sorts  for  construction  of  new  parts    (cell-walls 
and  protoplasm)  and  for  energy*    From  analogy  with  the 
known  behavior  of  the  carbohydrates,  and  from  experiment, 
it  may  be  concluded  that  the  amides  are  not  elaborated  in 
growing    parts,   but  rather  are  transferred  to  them  from 
parts  where  they  have  been  elaborated  from  sugar  and  in- 
organic  nitrogen    compounds^    or   from    parts    where    the 
already  elaborated  nitrogen  compounds  have  been  stored. 
These  compounds  are  stored,  in  most  cases,  in  insoluble 
forms    (e.  g.  the  proteids),  corresponding  in  this  physical 
quality  with  the  starches  and  oils,  and  associated  with  them 
in   the  cells  of  the  storing-tissues, — in  the  pith,  medullary 
rays,  the  cortical  and  pith  parenchyma  of  roots,  tubers,  and 
other  underground  parts,  in  seeds,  and  in  spores. 

The  complex  nitrogen  compounds  are  very  unstable  and 
are  doubtless  subject  in  the  plant  to  more  or  less  constant 
destruction  and  reconstruction.  The  destruction  of  complex 
nitrogenous  compounds  does  not  result  in  the  higher  plants 
in  the  excretion  of  waste  substances,  as  in  the  animals. 


NUTRITION  71 

The  plant  operates  more  economically.  From  the  simpler 
compounds  formed  by  breaking  down  a  highly  complex  or- 
ganic nitrogen  compound,  the  plant  reconstructs,  with  the 
necessary  additions,  complex  compounds  of  the  same  or 
similar  sorts.  In  green  plants  there  is  nothing  correspond- 
ing with  the  excretion  of  urea  by  higher  animals,  and  the 
view  long  held  that  asparagin  and  other  amides  retained 
within  the  plant-body  are  useless  substances,  like  the  ex- 
crementitious  matters  cast  off  by  higher  animals,  has  been 
shown  to  be  false.  Although  the  amides  are  undoubtedly 
formed  in  plants  in  consequence  of  the  breaking  down  of 
proteid  compounds,  they  must  be  regarded  as  also  represent- 
ing one  stage  in'  the  syntheses  of  proteids,*  The  greater 
economy  of  plants  consists  in  their  being  able  to  use  the 
amides,  and  probabty  also  other  and  simpler  substances, 
formed  during  the  destruction  of  the  molecules  of  proteids, 
whereas  animals  are  obliged  to  cast  off  large  quantities  of 
highly  elaborated  nitrogenous  substances,  f 

By  means  and  through  progressive  syntheses  not  defi- 
nitely known,  and  not  necessarily  the  same  in  plants  of 
different  species,  the  inorganic  nitrogen  compounds  (ni- 
trates )  occurring  in  soil  and  in  water  are  elaborated,  in  the 
body  of  the  plant,  to  complex  compounds  containing  car- 
bon, hydrogen,  oxygen,  and  nitrogen.  Amides,  of  which 
asparagin  (C^HgN^O,,)  is  the  most  familiar,  are  gradually 
built  up  from  nitrates,  ammonia  compounds,  etc.,  in  the 
plant-body.  The  amides  are  soluble,  diffusible  foods;  they 
may  be  moved  from  part  to  part  of  the  plant  as  needed, 
and  used  in  the  synthesis  of  more  complex  organic  nitro- 
genous compounds,  the  proteids,  etc.  Phosphorus  and  sul- 
phur are  subsequently  added  to  these  complex  nitrogenous 
compounds.  Substances  finally  are  formed  which  possess 
the  same  physical  and  chemical  structure  and  properties  as 
those  composing  the  living  protoplasm. 
The  incorporation  of  these  final  compounds,  which  are  at 

*  Schulze,  E.  f  her  den  Umsatz  der  Eiweisstoffe  in  der  lebenden  Pflanze. 
Zeitschr.  f.  physiol.  Chemie,  Bd.  24.  1898. 

t  When  I  was  a  student  in  his  laboratory,  Strasburger  once  said  to  me  : 
"  Es  ist  nicht  wie  im  Thierreich.  Die  Pflanzen  machen  nie  dummes  Zeug  I  " 


72  PLANT  PHYSIOLOGY 

the  summit  of  the  series  of  constructive  processes  carried  on 
by  the  plant,  into  the  living  protoplasm,  results  in  making 
them  parts  of  the  living  substance.  These  molecules,  the 
building  up  of  which  we  have  tried  to  trace,  do  not  them- 
selves necessarily  become  alive  when  they  are  made  parts  of 
the  living  substance,  and  yet  these  molecules  recently  formed 
and  others  similar  to  them  constitute  the  physical  basis  of 
life,  the  living  protoplasm.  The  new  molecules  incorporated 
into  and  made  parts  of  the  living  protoplasm  are  not  given 
any  new  or  peculiar  vital  force,  vis  naturae,  or  any  other 
occult  power,  but  are  so  distributed  in  space,  have  such  rate 
and  amplitude  of  movement,  that  under  the  conditions  pre- 
vailing in  the  cell — the  conditions  which  make  life  possible 
anywhere — they  behave  as  do  the  old  and  'are  as  the  old. 

The  majority  of  chlorophyll-containing  plants  depend, 
as  we  have  seen,  upon  nitrates  for  their  supply  of  nitrogen. 
In  the  laboratory,  ammonia  salts  and  even  ammonia 
vapor  may  be  made  to  serve  as  the  source  of  nitrogen,  but 
in  nature  they  are  not  the  immediate  source  for  green 
plants.  Besides  the  nitrates  and  ammonia,  animals  and 
plants  may  obtain  needed  nitrogen  from  three  sources, 
namely,  (#)  in  the  uncombined  state  from  the  air,  (/>) 
from  its  compounds  in  excrementitious  matter  of  animals, 
and  in  dead  bodies  of  animals  and  plants,  ( <- )  from  its  com- 
pounds in  living  bodies.  The  last  two  sources  (b  and  c) 
are  drawn  upon  by  dependent  organisms — animals  and 
saprophytic  and  parasitic  plants.  The  first  is  used  by  bac- 
teria, living  either  by  themselves  in  the  soil  or  associated 
with  higher  plants  in  special  outgrowths  of  their  roots. 

ROOT-TUBERCLE  PLANTS 

For  centuries  it  has  been  the  profitable  practice  of  farmers 
to  use  leguminous  crops  to  enrich  impoverished  soils.  A 
worn-out  field  wrill  more  rapidly  recover  its  fertility  if  sown 
to  clover  than  if  allowed  to  lie  fallow.  Sowing  to  clover  and 
plowing  the  crop  under  will  evidently  enrich  the  soil  more 
than  mowing  it  and  plowing  the  stubble  under,  but  even  the 
latter  is  better  for  the  soil  than  sowing  to  grass  and  plow- 


NUTRITION  73 

ing  the  whole  crop  under.  *  Chemical  analysis  of  soils  origi- 
nally alike  show  that  that  of  a  clover  field  gains  in  nitrogen 
as  well  as  in  carbon,  while  that  of  a  grass  field  gains  only 
in  carbon.  In  other  words,  plowing  a  grass-crop  under  re- 
turns to  the  soil  only  what  the  plants  took  from  it,  plus 
,the  amount  of  carbon-  absorbed  from  the  air.  On  the  other 
hand,  plowing  a  clover-crop  under  returns  to  the  soil  what 
the  plants  took  from  it,  plus  the  amount  of  carbon  ab- 
sorbed from  the  air,  and  nitrogen.  Where  did  the  nitrogen 
come  from?  Obviously  it  could  come  only  from  the  air. 

If  clover,  pea,  alfalfa,  or  other  leguminous  seed  be  sown 
in  sterilized  soil  of  known  composition,  no  increase  in  ni- 
trogen will  be  discovered  on  analyzing  the  soil  after  the 
crop  has  been  turned  under.  These  seeds  sown  in  soil  of 
exactly  the  same  composition,  unsterilized,  but  otherwise 
treated  in  the  same  way  before  and  during  cultivation, 
will  yield  a  larger  crop,  which  will  be  found  to  have  added 
nitrogen  to  the  soil.  These  seeds,  sown  in  soil  of  the 
same  composition,  sterilized  and  then  inoculated,  either  by 
the  addition  of  a  small  quantity  of  unsterilized  soil  or  of 
clear  water  first  sterilized  and  then  shaken  with  unsterilized 
soil,  will  produce  a  crop  as  large,  and  yield  as  large  a  gain 
of  nitrogen  in  the  soil,  as  those  sown  in  unsterilized  soil. 

From  these  experiments  t  the  only  possible  inference  is 
that  the  micro-organisms  of  the  soil,  and  not  the  legumi- 
nous plants  alone,  are  effectively  concerned  in  the  increase 

*  See  Year  Book  of  U.  S.  Dep't.  Agriculture  for  1897  and  other  publica- 
tions of  the  national  and  state  agricultural  departments  for  statistics  of 
relative  values  of  different  crops  as  "green  manures"  in  adding  to  the 
available  nitrogen  content  of  soil. 

t  For  details  see  Frank,  A.  B.    Die  Assimilation  des  freien  Stickstoffs 
durch  die    Pflanzenwelt.    Bot.  Zeitung,  Bd.  51,  1893,  and  elsewhere,  to 
which  references  are  given  in  this  paper. 
Also  various  papers  in  the  Deutsche  Landwirthschaftliche  Presse, 

"      "    "    Landwirthschaftliches  Jahrbuch, 
"  "  "      "    "    Annales  Agronomiques, 

"  "  "      "    "    Year  Book  U.  S.  Department  of  Agriculture, 

"  "  "      "    "    Reports    of  "    "  "          " 

"          "  "      "    "  "  Agricultural  Experiment  Stations 

of  different  states,  etc. 


74  PLANT  PHYSIOLOGY 

in  the  nitrogen  content  of  the  soil.  The  effectiveness  of  the 
micro-organisms,  however,  may  be  of  one  of  two  sorts; 
either  they  may  themselves  absorb  and  elaborate  the  free 
nitrogen  of  the  air,  or  they  may  stimulate  the  leguminous 
plants  to  do  so.  An  acquaintance  with  the  nature  of  their 
association  with  the  leguminous  plants  and  with  the  micro- 
organisms themselves  is  a  necessary  preliminary  to  an  in- 
telligent discussion  of  this  question. 

On  the  roots  of  leguminous  plants*  develop  nodules  of 
various  sizes,  smooth  or  convoluted.  The  roots  of  plants 
only  a  few  weeks  old  begin  to  lose  their  even  contour  and 
uniform  diameter,  swellings  occur  at  irregular  intervals,  and 
these  increase  in  size  very  considerably  until  the  plant 
fruits.  These  nodules  or  root-tubercles,  white  or  rose- 
colored,  are  composed  mainly  of  thin-walled  parenchyma 
cells,  enclosing  small  intercellular  spaces,  and  directly  ad- 
joining the  vascular  tissues.  Sections  of  young  tubercles 
show  large  cells  with  dense,  coarsely  granular  contents, 
which,  on  closer  examination,  fall  under  two  distinct  heads, 
— the  protoplasm  of  the  cells,  and  slender  rods  which  prove 
to  be  living  bacteria.  In  older  tubercles  many  of  the  bac- 
teria have  enlarged  and  degenerated,  are  of  irregular  Y  and 
T  forms.  In  still  older  ones,  the  bacteria  are  dead.  In 
other  cells  than  those  composing  the  tubercles  no  bacteria 
are  to  be  found.!  When  the  plant  begins  to  fruit,  the  tuber- 
cles lose  their  plump  and  even  appearance,  become  emptied 
and  shrivelled.  Their  cells  then  contain  only  fragments  of 
the  deformed  and  dead  bacteroids  (as  the  "involution"  or 
degenerate  forms  are  called )  and  only  a  few  intact  and  liv- 
ing rods.  These  last,  set  free  in  the  soil  by  the  complete 
breaking  down  of  the  walls  of  the  tubercles,  survive  until 

*  Also  on  those  of  alder  (see  Hiltner  in  Versuchsstationen,  Bd.  46, 
1896),  Eleagnus  (see  Nobbe  in  Versuchsstationen,  Bd.  41,  1892,  and 
Bd.  45,  1894)  and  Podocarpus  (see  Janse  in  Annales  du  Jardin  Botan. 
de  Buitenzorg,  Bd.  14,  1896). 

f  Contrary  to  Frank's  assertion  (Uber  die  Pilzsymbiose  der  Legumi- 
nosen.  Landwirthschaftliche  Jahrbiicher,  Bd.  19,  1890),  Zinnser  (fiber  das 
Verhalten  von  Bakterien,  insbesondere  von  Knollchenbakterien,  in  leben- 
den  pflanzlichen  Geweben.  Jahrb.  f.  wiss.  Bot.,  Bd.  30,  1897)  found  bac- 
teria in  the  tubercles  exclusively. 


NUTRITION  75 

the  following  season  and  then  accomplish  the  infection  of 
the  roots  of  the  new  crop.  Infection  takes  place  through 
root-hairs  attacked  and  entered  by  these  bacteria.*  The 
bacteria  may  be  grown  in  artificial  culture  media  inoculated 
from  tubercles.  In  such  cultures  there  is  an  appreciable 
increase  in  nitrogen.! 

Leguminous  plants  will  grow  in  sterilized  soil  containing 
nitrates  in  forms  and  in  quantities  suitable  for  the  successful 
cultivation  of  other  plants,  but  under  the  conditions  em- 
ployed in  experimenting  they  do  not  produce  crops  so  good 
as  when  grown  in  unsterilized  soil.  The  following  figures 
will  indicate  the  benefit  they  derive  from  association  with 
the  proper  bacteria  :  J 

Per  culture-jar,  each  holding  two  plants  of  Lupinus 
lutews — 

I.      WITH  TUBERCLE   FORMATION 

Yield  of  dry  substance  Weight  of  N  Weight  of  N  supplied  Gain  or  loss 

in  grams.  therein.  in  seed,  soil,  and  water.  of  N. 

(a)38.919  0.998  0.022  +  0.975 

(6)33.755  0.981  0.023  +  0.958 

II.      WITHOUT   TUBERCLE   FORMATION 

Yield  of  dry  substance  Weight  of  N  Weight  of  N  supplied  Gain  or  loss 

in  grams.  therein.  in  seed,  soil,  and  water.  of  N. 

(c)  0.989  0.016  0.020  —0.004 

(J)0.828  0.011  0.022  —0.009 

(d)  was  watered  with  sterilized  lupin-soil  water  (40 
grams ) . 

(#  and  b)  watered  with  unsterilized  lupin-soil  wrater  (40 
grams ) . 

( c )  was  watered  with  sterilized  ( tap  ? )  water. 

It  must  be  borne  in  mind,  however,  that  though  the  legu- 
minous plants  may  appear  to  profit  by  such  association 
with  bacteria,  these  results  are  derived  from  experiments 

*  Peirce,  G.  J.  Root-tubercles  of  Bur  Clover  and  of  some  other  legumi- 
nous plants.  Proc.  Cal.  Acad.  Sciences,  Botany,  vol.  II.,  1902. 

t  Maze.  Fixation  de  1'azotte  libre  par  le  bacille  des  nodosites  des  Legu- 
mineuses.  Annales  de  Tlnstitut  Pasteur,  t.  XI.,  1897. 

|  Hellriegel  und  Wilfarth.  Erfolgt  die  Assimilation  des  freien  Stickstoffs 
durch  die  Leguminosen  unter  Mitwirkung  niederer  Organismen?  Ber.  d. 
Deutsch.  Bot.  Gesellschaft.  Bd.  VII.,  1889. 


76  PLANT  PHYSIOLOGY 

under  conditions  wholly  unnatural  to  all  the  plants  experi- 
mented upon,  and  must  therefore  be  taken  with  reserve. 
Leguminous  plants  grown  in  glass  vessels  undeniably  do 
better  when  their  roots  are  infected  by  bacteria  than  when 
their  roots  are  sterile,  but  it  has  riot  been  proved  that 
leguminous  plants  do  better  with  their  roots  infected  than 
with  sterile  roots  when  they  are  grown  where  their  roots 
can  be  properly  aerated.  Infected  LeguniinostB  benefit  the 
soil  more  than  do  sterile  ones,  but  what  the  gain  to  the 
plants  themselves  may  be,  remains  to  be  shown,  for  the 
bacteria  are  plainly  parasites.* 

The  bacteria  found  in  and  causing  the  root-tubercles  of 
the  Leguminosie,  Eleagnus,  etc.,  have  not  }^et  been  isolated 
from  the  soil  and  are  known  only  in  the  tubercles  and  in 
cultures  inoculated  from  tubercles.  The  isolation  of  another 
species  of  bacteria  which  fix  free  atmospheric  nitrogen 
(Clostridium  Pasteurianum)  has,  however,  been  accom- 
plished by  Winogradsky.f  Other  species  will  doubtless  be 
found  in  the  cultivated  and  undisturbed  soils  of  field  and 
forest.  The  green  and  blue-green  algse  growing  on  the  soil 
were  suspected  of  being  able  to  fix  uncombined  nitrogen, 
but  it  has  been  demonstrated  that  they  cannot  do  this 
alone.  |  It  may  be  that  nitrogen-fixing  soil-bacteria  and 
low  algPB  live  together  in  an  association  similar  to  that  of 
bacteria  and  leguminous  plants.  § 

The  number  of  organisms  which  can  use  free  nitrogen  will 
undoubtedly  be  found  to  be  small,  for  in  the  present  balance 
of  nature  little  more  nitrogen  need  be  added  to  the  soil 
than  is  yearly  returned  to  it  in  the  excrementitious  matters 
and  in  the  dead  bodies  which  fall  upon  it.  In  these  waste 
matters  are  organic  nitrogen  compounds  upon  which  de- 

*  Peirce,  G.  J.     Loc.  cit. 

t  Winogradsky,  S.  Sur  1'assimilation  de  1'azote  gazeux  de  I'atmosphere 
par  les  microbes.  Comptes  Rendus,  t.  116.  1893  t.  118,  1894;  also 
Archives  des  Sciences  Biologiques  St.  Petersburg,  Bd.  3,  1895. 

J  Kossowitsch.  P.  Untersuchungen  iiber  die  Frage  ob  die  Algen  freien 
Stickstoff  assimiliren.  Botanische  Zeitung,  1894. 

§  Pfeffer,  W.  Pflanzenphysiologie,  Bd.  I.,  p.  386.  Engl.  transl.  I.,  p. 
396.  See  also  Kruger  und  Schneiderwind  in  Landw.  Jahrb.,  Bd.  29,  Nos. 
4  and  5,  pp.  771,  804  1900. 


NUTRITION 

pends  the  existence  of  a  very  large  number  of  org* 
which  break  down  these  complex  substances  to  simple 
nitrates,  the  nitrogen  compounds  which  alone  are  useful  to 
the  majority  of  green  plants. 

The  organisms  accomplishing  these  decompositions  take 
into  their  own  bodies  nitrogenous  and  non-nitrogenous  car- 
bon compounds  elaborated  by  other  and  higher  organisms, 
reconstruct  and  assimilate  some  of  the  substance,  making  it 
a  living  part  of  themselves,  decomposing  the  rest  by  physio- 
logical oxidation  or  by  anaerobic  respiration  in  order  to 
obtain  energy.  In  the  farmer's  manure  pile  there  are  count- 
less dependent  organisms  which  fall  into  separate  species, 
I  easily  conceivable  but  most  difficult  to  isolate.  On  the  sur- 
face of  the  pile  are  fungi  and  bacteria  which  respire  aerobi- 
cally  and  attack  mainly  the  non-nitrogenous  matters,  the 
cellulose  walls  in  the  fragments  of  straw,  and  other  vege- 
table remains.  Within  the  pile  are  the  anaerobic  organisms, 
the  first  set  living  on  the  proteids  and  amides  contained  in 
the  animal  and  vegetable  matters,  building  up  their  own 
body  substance  from  some  of  these  and  decomposing  others. 
Living  upon  the  decomposition  products  and  upon  the  dead 
bodies  of  the  first  is  the  second  set,  which  similarly  build  up 
and  break  down.  A  third  set  subsists  on  the  products  and 
upon  the  remains  of  the  second;  and  so  on  down  to  the 
nitrite  and  nitrate  bacteria  wrhich  oxidize  ammonia  (the 
ultimate  product  of  a  great  number  of  decompositions )  to 
nitrites  and  these  to  nitrates  respectively. 

When  the  highly  complex  protoplasmic  substances  of  ani- 
mals and  plants,  upon  which  few  organisms  can  live,  are 
broken  down  to  ammonia,  water,  carbon-dioxide,  etc.,  and 
the  ammonia  is  oxidized  to  nitric  acid,  green  plants  can 
begin  again  the  constructive  processes  which  end  in  the  for- 
mation of  living  protoplasm  from  inorganic  nitrogen  com- 
pounds of  the  simplest  sort.  Thus  we  have  the  cycle  of 
nitrogen  in  the  physiology  of  living  organisms. 

A  number  of  plants— some  leading  an  apparently  inde- 
pendent existence— are  forced,  or  at  least  find  it  advanta- 
geous, to  add  to  their  supply  of  nitrogenous  matters  by 
taking  into  their  bodies  organic  (that  is,  carbon)  com- 


78  PLANT  PHYSIOLOGY 

pounds  of  nitrogen.  These  plants  fall  into  one  of  three 
classes,— the  humus  plants,  the  carnivorous  plants,  and  the 
parasites. 

HUMUS  PLANTS 

The  humus  plants  live  in  soil  containing  a  large  amount 
of  organic  matter,  mainly  of  vegetable  origin,  in  a  more  or 
less  decomposed  condition.  Besides  these  organic  remains, 
living  saprophytic  fungi  form  an  important  constituent  of 
humus.  Owing  to  the  very  diverse  composition  of  humus 
soils  (loam,  leaf-mould,  etc.),  it  is  very  difficult  to  deter- 
mine what  substances  are  absorbed  from  them  by  plants, 
but  since  the  majority  of  the  humus  plants  contain  either 
no  chlorophyll  or  only  a  little,  they  must  absorb  elaborated 
non-nitrogenous,  as  well  as  nitrogenous,  carbon  compounds. 
These  soils  consist  largely  of  substances  insoluble  or  only 
slightly  soluble  in  water.  It  is  very  probable,  therefore, 
that  there  is  a  solvent  action  exerted  by  the  underground 
parts  of  humus  plants.  The  fact  that  even  the  immediately 
soluble  constituents  of  humus  soils  diffuse  only  slowly, 
strengthens  the  supposition  that  some  if  not  all  of  the 
humus  plants  dissolve  the  nutritious  substances  upon  which 
their  life  depends.  Those  growing  in  humus  soils  containing 
little  soluble  food-material,  but  incapable  of  secreting  acids 
or  other  solvents,  depend  upon  other  organisms  which  can 
exert  a  solvent  action  and  with  these  they  live  in  more  or 
less  intimate  association. 

The  plants  most  active  in  converting  the  insoluble  nitro- 
genous substances  in  lifeless  remains  of  higher  organisms 
into  soluble  and  hence  more  generally  available  compounds, 
are,  as  we  have  seen,  the  bacteria.  Almost  equally  impor- 
tant are  the  saprophytic  fungi — toadstools  and  similar 
plants  common  in  all  sufficiently  moist  humus.  These  fungi 
and  the  bacteria,  in  nourishing  themselves,  prepare  the 
materials  needed  and  used  by  other  humus  plants  growing 
with  them.  One  step  farther  some  of  the  higher  humus 
plants  go.  Instead  of  living  merely  close  to  the  lower 
humus  organisms,  upon  the  activities  of  which  they  are  de- 
pendent, they  come  into  actual  contact  with  these,  their 


NUTRITION  79 

roots  becoming  invested  or  even  penetrated  by  them.  These 
associations,  described  by  Frank*  most  fully,  are  still  too 
little  understood  to  enable  one  to  determine  what  parts  are 
played  by  the  members,  or  to  decide  whether  the  associa- 
tion is  of  mutual  advantage  or  not. 

The  association  of  filamentous  fungi  with  the  roots  of 
higher  plants — called  Mycorhiza  by  Frank* — is  not  confined 
to  those  poor  in  chlorophyll  (e.  g.  Neottia)  or  devoid  of  it 
( e.  g.  Monotropa ) ,  but  occurs  also  in  a  considerable  number 
of  green  plants  (e.  g.  many  Orchidaceae,  Ericaceae,  Cupuli- 
ferse,  Piuus,  etc.).  This,  coupled  with  the  fact  that  so  little 
is  known  of  the  chemistry  of  nutrition  in  these  associations, 
renders  it  impossible  to  draw  any  general  conclusions  re- 
garding the  work  accomplished  by  the  fungi.  Those  closely 
investing  and  making  felt-like  sheaths  over  the  roots  of 
certain  forest  trees  (e.g.  beech  and  pine)  cannot  be  sup- 
posed to  furnish  the  larger  member  of  the  association  with 
non-nitrogenous  carbon  compounds  from  the  soil,  for  these 
it  can  elaborate  in  abundance  in  its  own  green  leaves. 
Mineral  salts  and  appropriate  nitrogen  compounds  the  fun- 
gus may  supply,  first,  because  of  its  ability,  by  reason  of 
its  smaller  size,  to  branch  more  finely  and  spread  more 
widely  among  the  soil-particles  than  can  the  roots ;  second, 
because  of  its  decomposing  action  upon  insoluble  nitroge- 
nous remains  in  the  soil ;  and  third,  because  it  may  elaborate 
or  oxidize  the  otherwise  useless  ammonia  compounds. f 

It  is  claimed  that  the  fungi  may  be  of  additional  ad- 
vantage because  roots  invested  by  them  branch  more  pro- 
fusely than  naked  ones,  and  hence  are  intimately  in  contact 
with  more  soil  particles  and  have  a  larger  absorbing  sys- 
tem. J  Neither  in  this  claim,  nor  in  the  observation  that 
root-hairs  are  largely  absent  from  the  roots  of  green  as  well 

*  Frank,  A.  B.  Lehrbuch  der  Botanik,  Bd.  I.,  1892.  Also  Percy  Groom 
in  Annals  of  Botany,  Vol.  9,  1895.  See  also  Stahl,  E.  Der  Sinn  der 
Mycorhizenbildung.  Jahrb.  f.  wiss.  Bot.,  Bd.  34,  1900.  MacDougal,  D. 
T.  Symbiotic  saprophytism.  Annals  of  Bot.,  XIII.,  1899. 

t  Frank,  A.  B.  Die  Krankheiten  der  Pflanzen,  2te  Aufl.,  1895.  Also 
die  Bedeutung  der  Mycorhiza-Pilze  fiir  die  gemeine  Kiefer.  Forstwissen- 
schaftliches  Centralblatt,  XVI.,  1894. 

t  Stahl,  E.    Loc.  cit. 


80  PLANT  PHYSIOLOGY 

as  other  plants  .associated  with  Mycorhiza  fungi,  are  there 
any  grounds  for  assuming  that  we  have  other  than  patho- 
logical conditions  due  to  the  interference  of  the  fungi  with 
the  normal  independent  habits  of  higher  plants.  So  far  as 
the  chlorophyll-containing  plants  are  concerned,  Mycorhiza 
seems  an  affliction  rather  than  a  blessing,  despite  the  claims 
of  Frank  and  of  Stahl.  Frank  says  the  growth  of  seedlings 
of  beech  and  pine,  cultivated  in  sterilized  humus  soil,  is  less 
than  of  other  seedlings  of  the  same  sort  in  the  same  soil 
unsterilized.  Stahl  shows  that  plants  ordinarily  free  from 
Mycorhiza  grow  better  in  sterilized  than  in  unsterilized 
humus.  It  remains  for  experiment  to  show  whether  beech, 
pine,  and  other  plants  with  fungi  usually  on  or  in  their 
roots  grow  better  in  sterilized  fertile  soil  free  from  humus 
or  in  humus  which  has  not  been  sterilized.  Some  green 
plants  are  strictly  humus  plants,  refusing  to  grow  in  other 
soils,  but  one  cannot  now  conclude  from  this  that  they  are 
dependent  upon  the  fungi,  rather  than  upon  any  other  con- 
stituent of  the  humus.  Sterilizing  a  humus  soil  causes 
changes  in  the  physical  arid  chemical  conditions  of  many 
nitrogenous  matters  in  the  humus.  Some  plants  are  able  to 
accommodate  themselves  to  these  changes,  others  are  not. 
Sterilizing  the  humus  causes  the  death,  not  only  of  the 
Mycorhiza  fungi,  but  also  of  the  nitrifying  bacteria  and  of 
those  other  bacteria  and  fungi  which  directly  or  indirectly 
produce  ammonia  from  the  organic  nitrogen  compounds. 
These  changes  must  profoundly  disturb  the  balance  of  ac- 
tivities in  the  soil.  It  is,  therefore,  much  easier  to  under- 
stand the  benefit  derived  by  the  fungi  from  their  intimate 
association  with  the  roots  of  plants  able  to  manufacture 
food  for  themselves,  than  to  be  convinced  that  the  indepen- 
dent green  plants  greatly  benefit  by  association  with  de- 
pendent ones.  However,  the  subject  deserves  further  investi- 
gation. 

The  association  of  high  and  low  colorless  plants  in  My- 
corhiza is  different  only  in  degree,  not  in  kind,  from  their 
simultaneous  occurrence  in  all  places  where  there  are  highly 
elaborated  nitrogenous  and  non-nitrogenous  materials.  As 
the  nitrate  bacteria  can  work  only  upon  nitrites  formed 


NUTRITION  81 

from  ammonia  by  the  nitrite  bacteria,  and  the  sulphur 
bacteria  can  live  only  where  sulphuretted  hydrogen  is  abun- 
dantly set  free  either  by  organisms  or  in  sulphur  springs,  so 
in  the  humus  soils  some  organisms  live  upon  the  simpler 
products  of  their  neighbors.  It  is  easy  to  conceive  that  more 
than  neighborhood  nearness  might  be  advantageous  to  some, 
and  that  these,  gradually  growing  together,  might  form 
associations,  mutually  though  perhaps  not  equally  benefi- 
cial. The  penetration  of  small  organisms,  or  small  parts  of 
organisms,  into  and  even  through  the  living  cells  of  others, 
is  not  necessarily  fatal  to  the  penetrated  cells,  as  is  shown 
by  the  symbiotic  association  of  algae  enclosed  in  the  cells 
of  certain  Infusoria,  and  even  by  some  parasitic  associa- 
tions. * 

In  all  of  these  cases,  however,  we  have  no  new  physio- 
logical principles.  The  nutrition  is  fundamentally  the  same, 
the  food-materials  are  acquired  and  elaborated,  the  foods 
are  assimilated  and  incorporated,  in  the  same  way  in  all 
organisms,  although  the  sources  of  food  may  be  different  in 
different  cases. 

CARNIVOROUS  PLANTS 

The  carnivorous  plants  have  been  exploited  especially 
by  "ecologists,"  students  of  the  adaptations  of  plants  to 
their  surroundings,  and  their  writings  furnish  the  proper 
sources  for  more  general  information  regarding  them  ;t  but 
the  peculiarities  of  their  nutrition  are  still  within  the  prov- 
ince of  pure  physiology.  These  extraordinary  plants  may 
be  divided  into  two  classes:  first,  those  which  forcibly 
capture,  and  second,  those  which  simply  entrap,  their  prey. 
These  classes  may  be  represented  respectively  by  Dion&a, 

*  For  example,  the  cases  cited  by  De  Bary  (Morphology  and  Biology  of 
the  Fungi,  Mycetozoa,  and  Bacteria,  Eng.  transl.,  p.  392,  3)  and  the  sur- 
vival of  chlorophyll-containing  cells  of  the  host  even  when  penetrated 
by  cells  from  the  haustoria  of  Cuscuta  (Peirce,  Annals  of  Botany,  Vol. 
vii.,  p.  308,  1893). 

\  See  Darwin's  Insectivorous  Plants,  Goebel's  Pflanzenbiologische  Schil- 
derungen  (Insektivoren),  Kerner  and  Oliver's  Natural  History  of  Plants 

((Vol.  I.,  part  I.),  Cohn's  Die  Pflanze  (Bd.  II.,  Insektenfressende  Pflanzen), 
Ludwig's  Lehrbuch  der  Biologic  der  Pflanzen,  etc.,  etc. 


82  PLANT  PHYSIOLOGY 

Droseru,  Pinguicula,  and  by  Sarrace nhi,  Darlingtonia,  the 
Nepenthes,  and  Utricularia.  Of  these  Di'osem,  Sarraceiria, 
and  Utricularia  are  most  familiar  and  will  sufficiently  illus- 
trate the  principles  concerned. 

Drosvm,  the  "  Sun-dew"  of  northern  bogs,  is  a  low  annual, 
with  nearly  horizontally  expanded,  round,  or  elongated 
leaves,  characterized  by  peculiar  columnar  outgrowths 
from  the  upper  surface.  The  chlorophyll  of  the  leaves  is 
usually  masked  by  the  red  cell-sap  of  the  superficial  cells  and 
by  the  slender  outgrowths.  These  last  are  multicellular, 
traversed  for  about  half  their  length  by  a  single  vertical 
vascular  bundle,  and  covered  except  on  the  top  by  ordinary 
epidermal  cells  with  cutinized  walls.  The  free  ends  of  the 
columnar  structures  are  enlarged  and  globular,  glandular, 
and  covered  by  a  glistening,  sticky,  syrupy,  more  or  less 
sweet  secretion.  Attracted  by  the  unusual  color  and  the 
glistening  surface  of  these  leaves,  small  insects  alight  upon 
them,  taste  the  sweet  secretion,  and  while  they  feed  upon  it 
are  detained  by  its  stickiness.  The  weight  and  movements 
of  the  insect  induce  movements  in  the  hairs  adjacent  but 
untouched  and  in  the  blade  of  the  leaf  itself,  as  well  as  in 
the  hairs  with  which  it  is  in  contact.  Unless  the  insect 
is  powerful  enough  to  break  away  from  the  sticky  surface, 
it  presently  comes  into  contact  with  other  hairs  and  sticks 
to  them  also.  Finally  the  blade  of  the  leaf  bends  upward 
and  the  prey  becomes  enclosed  between  it  and  the  many  hair- 
like  tentacles  bending  over  upon  it  from  all  sides.  The 
continued  mechanical  irritation  of  the  hairs  causes  a  more 
abundant  secretion  from  the  glandular  ends ;  but,  as  Darwin 
has  shown,*  besides  the  mechanical  there  is  also  a  chemical 
irritation,  and  this  latter  induces  a  change  in  the  composi- 
tion of  the  secretion.  Non-nitrogenous  material — a  stone  or 
a  piece  of  wood — of  similar  size  and  weight  will  not  induce 
the  same  response  as  an  insect,  a  piece  of  meat  or  of  hard- 
boiled  egg,  plainly  showing  that  upon  the  chemical  composi- 
tion of  the  irritating  body  depends  in  part  the  nature  of 
the  irritation  and  of  the  response. 

The  continued  contact  of  highly  elaborated  organic  nitro- 
*  Darwin,  Charles.  L.  c. 


NUTRITION  83 

gen  compounds,  largely  proteids  and  insoluble,  is  followed 
by  the  secretion  from  the  glandular  ends  of  the  hairs  of  a 
peptonizing  enzym  which  attacks  the  nutritious  substances 
and  dissolves  them.  These  solutions  are  absorbed  into  the 
plant  through  the  cells  secreting  the  enzym,  diffuse  from 
them  into  other  cells,  and  are  conducted  away  by  the  vas- 
cular bundle.  After  all  the  nutritious  substances  in  the 
captured  insect  have  been  digested  (dissolved)  and  ab- 
sorbed, the  leaf  unrolls,  again  becomes  flat,  the  tentacles 
loose  their  hold  and  become  straight,  the  chitinous  shell 
and  the  other  useless  remnants  are  exposed,  dry.  and  blow 
away.  The  leaf  is  now  ready  to  capture  another  insect.  It 
is  reported  that  each  leaf  has  a  digestive  capacity  for  two 
or  three  flies. 

When  Drosera  is  able,  by  capturing  and  digesting  insects, 
to  supplement  the  nitrogenous  food  which  it,  like  other 
plants,  elaborates  from  the  nitrates  absorbed  from  the  soil 
and  the  sugars  made  in  its  own  leaves,  it  attains  a  larger 
size  and  produces  more  and  better  seeds  ( other  things  being 
equal)  than  when  it  must  depend  solely  upon  the  complex 
nitrogenous  foods  made  by  itself. 

It  has  been  claimed  by  Tischutkin,  *  and  the  view  has  been 
somewhat  generally  accepted  without  due  investigation, 
that  the  peptonizing  enzym  is  secreted  mainly,  if  not  wholly, 
by  bacteria  symbiotically  associated  with  Drosera  on  its 
tentacles,  and  not  by  the  gland  cells  of  the  tentacles.  There 
can  be  no  question  of  the  constant  presence,  under  natural 
conditions,  of  bacteria  on  the  tentacles,  and  it  would  be  re- 
markable if  there  were  none  among  these  which  formed  a 
peptonizing  enzym.  However,  these  bacteria,  peptonizing 
and  other,  are  no  more  symbiotically  associated  with  Dro- 
sera  and  no  more  concerned  in  the  digestion  of  its  insect 
food  than  the  bacteria  of  the  human  mouth  and  digestive 
tract  are  symbiotically  associated  with  man  and  aid  in  his 
digestive  processes.  In  both  cases  we  have  to  do  with  bac- 
teria unavoidably  and  constantly,  but  also  accidentally  and 

*  Tischutkin  N.  A  Russian  paper  reviewed  in  Botan.  Centralblatt 
Bd.  50.  p.  304-j-.  1892 ;  also  a  later  paper  reviewed  in  Botan.  Central- 
blatt  Bd.  53.  p.  322.  1893. 


84  PLANT  PHYSIOLOGY 

independently,  present.    They  are  not  more  intimately  as- 
sociated with  the  higher  organism. 

The  same  hypothesis  has  been  extended  to  the  digestion 
which  takes  place  in  the  pitcher-like  leaves  of  Nepenthes,  and 
with  much  more  justice  to  the  decompositions  in  the  similar 
leaves  of  Sarracenia.  For  Nepenthes,  Goebel*  and  Vinest 
have  proved  the  presence  in  the  pitchers  of  an  enzym  capa- 
ble of  digesting  proteid  matters.  Goebel  J  says  that  the 
Sarracenias  and  Darhngtonia  secrete  neither  an  enzym  nor 
a  substance  which  checks  decay ;  that  is,  they  do  not  them- 
selves digest  the  bodies  of  insects,  but,  on  the  other  hand, 
they  do  not  prevent  the  decomposition  of  these  by  living 
organisms  contained  in  the  pitchers.  The  Sarracenias  and 
Darlingtoma,  like  Drosera,  inhabit  northern  bogs,  the  soils 
of  which  are  relatively  poor  in  nitrogen.  The  leaves  are 
large,  erect  or  inclined,  pitcher-shaped,  holding  often  con- 
siderable volumes  of  water.  Owing  to  the  peculiar  form,  the 
slippery  inner  surface,  and  its  downward-pointing  hairs,  the 
mouth  of  a  pitcher  is  an  alighting-place  as  uncertain  as  it 
is  natural  for  flying  insects.  They  fall  down  into  the  pitch- 
er; their  escape  is  prevented  by  the  shape,  slippery  sur- 
face, and  hairiness  of  the  inside  of  the  pitcher ;  finally  they 
die,  either  by  drowning,  or  of  starvation  and  exhaustion 
from  their  futile  efforts  to  get  out.  They  now  decay,  bac- 
teria feeding  upon  their  dead  bodies  liberating  soluble  or- 
ganic nitrogen  compounds  which  are  absorbed  through  the 
walls  of  the  pitchers.  If  Goebel's  conclusions,  based  on 
investigations  carried  on  in  the  Botanic  Garden  at  Marburg, 
are  confirmed  by  equally  reliable  investigations  of  these 
plants  in  their  native  homes,  they  may  constitute  an  inter- 
esting case  of  an  association  mutually  beneficial;  but  it 
seems  hardly  a  sufficient  reason  for  the  formation  of  such 
extremely  modified  leaves  as  these  pitchers,  that  they 
serve  only  as  traps  for  insects,  culture-tubes  for  bacteria, 

*  Goebel,   K.    Pflanzenbiologische  Schilderungen ,  2ter  Theil,  p.   186  et 

SPf], 

f  Vines,  S.  H.    The  proteolytic  enzyme  of  Nepenthes.  Annals  of  Botany, 
Vol.  XI.,  1897.  Vol.  XII.,  1898. 
J  Goebel    K.    L.  c.   p.  170. 


NUTRITION  85 

and  absorbers  of  the  unused  products  of  these  micro-organ- 
isms. It  would  seem  much  more  probable  that,  under  their 
normal  conditions,  the  walls  of  these  pitchers,  as  of  the 
Xepenthes,  secrete  an  enzym  which  digests  the  bodies  of  en- 
trapped insects.  If  this  be  true,  the  bacteria  inevitably 
present  do  not  aid  the  plant,  they  simply  rob  it  of  food  which 
it  can  itself  digest  and  afterwards  absorb  and  assimilate. 

Most  of  the  Utricularias  are  aquatics,  the  peculiar  sub- 
merged leaves  of  which  entrap  small  crustaceans.  It  is  still 
undetermined  whether  the  crustaceans  are  killed  and  di- 
gested, *  or  whether  they  live  on  indefinitely.  When  they  die 
naturally  their  bodies  become  decomposed  by  the  water 
bacteria  invariably  present  in  the  bladder-like  leaves.  At  all 
events,  the  excreta,  containing  considerable  quantities  of 
organic  nitrogenous  matter,  cannot  fail  to  be  directly  or 
indirectly  useful  to  the  plant  which  harbors  the  animals 
producing  them.  The  excreta  may  contain  immediately 
available  substances ;  the  more  refractory  may  first  undergo 
chemical  transformation  by  water-bacteria ;  finally  the  dead 
bodies  and  the  lifeless  excreta,  falling  a  prey  to  bacterial 
activity,  become  available  to  the  Utricularia ,  which  profits 
accordingly.  .  I 

PARASITES 

A  considerable  number  of  plants  of  the  most  diverse  sorts, 
from  the  simplest  to  the  most  highly  developed,  are  able  to 
live  actively  only  upon  or  in  the  living  bodies  of  other  or- 
ganisms. They  may  be  able  to  survive  in  the  resting  condi- 
tion entirely  independently,  but  in  this  regard  their  seeds, 
spores,  and  cysts  are  like  all  others.  For  active  vegetation 
they  need  their  proper  hosts.  The  relation  of  the  parasite 
to  the  host  is  not  in  all  cases  simply  that  of  the  fed  to  the 
living  organism  which  nourishes  it.  The  host  may  do  much 
more  than  supply  the  parasite  with  food:  it  may  give  it 
mechanical  support,  it  may  stimulate  it  to  grow  in  charac- 
teristic forms,  it  may  assist  in  the  dissemination  of  its  off- 
spring, it  may  protect  it  in  a  variety  of  ways.  In  all  of 
these  respects  the  parasite  is  benefited. 

*  Goebel    K.    /..  ,-.    p.  173. 


86  PLANT  PHYSIOLOGY 

In  certain  instances  it  is  claimed  that  parasitism  is  ad- 
vantageous not  only  to  the  parasite  but  also  to  the  host. 
These  cases  must  be  examined  separately.  Pure  parasitism 
is  beneficial  only  to  the  parasite.  Whatever  may  be  our 
views  as  to  the  origin  of  parasitism,  we  must  admit  that 
parasitic  associations  are  entered  into  each  season  only 
because  the  parasite  is  a  dependent  organism,  incapable  of 
elaborating  its  own  complex  foods  from  simple  compounds. 
The  parasite  may  be  wholly  dependent,  taking  from  its  host 
all  the  food  it  needs,  or  it  may  be  only  partly  dependent, 
taking  only  certain  kinds  of  food.  In  either  case  the  foods 
absorbed  are  worked  over,  assimilated,  incorporated  or  con- 
sumed, by  the  parasite  itself.  No  food  is  absorbed  by  the 
parasite  in  forms  which  it  can  use  without  modification  for 
building  its  own  body. 

The  nutrition  of  parasites  differs  from  that  of  other  or- 
ganisms only  in  certain  stages  of  the  process,  and  not  fun- 
damentally even  in  these.  Instead  of  absorbing  from  water, 
soil,  and  air  the  raw  materials  which  must  be  elaborated 
into  foods,  the  parasite  absorbs  from  its  host  matters  al- 
ready elaborated  by  its  host.  The  means  of  absorption  are 
the  same  in  parasites  as  in  other  organisms  (see  Chapter 
IV.). 

Parasitism  consists  essentially  in  the  absorption  from  a 
living  organism  of  more  concentrated  solutions  of  more 
highly  elaborated  food-materials  or  foods  than  can  other- 
wise be  obtained.  Using  the  division  of  the  process  of  nutri- 
tion proposed  on  page  41,  into,  first,  the  absorption,  and 
second,  the  combination  of  food-materials,  third,  the  assimi- 
lation, and  fourth,  the  incorporation  of  foods,  we  see  that 
the  parasite  differs  from  the  self-sustaining  and  independent 
green  plant  only  in  omitting  the  second  stage  of  the  process, 
absorbing  from  its  host  the  food-materials  or  foods  already 
elaborated,  which  an  independent  plant  would  elaborate  for 
itself.  To  make  this  clearer,  let  us  examine  a  few  instances. 

Among  higher  plants,  perhaps  the  simplest,  in  other  words, 
the  least  complete  form  of  parasitism  is  that  of  the  Euro- 
pean and  American  mistletoes,  Viscum  album  and  Phorn- 
dendron  villosun).  The  mistletoes  are  green  perennials  con- 


NUTRITION  87 

taining  in  their  leaves  and  in  the  cortex  of  the  young  and 
even  older  branches,  an  abundance  of  chlorophyll  in  nor- 
mally effective  chloroplastids.  They  are  plants  which  can 
and  do  photosynthetically  manufacture  from  water  and 
carbon-dioxide  all  the  non-nitrogenous  food  which  they 
need.  Elevated  far  above  ground  on  the  branches  of  oaks, 
poplars,  apples,  etc.,  they  must  draw  from  their  hosts  the 
water  which  they  combine  with  the  carbon-dioxide  absorbed 
by  their  own  leaves.  Since  the  water  in  plants  is  a  dilute 
solution  of  a  large  variety  of  matters,  chiefly  mineral,  the 
mistletoe  absorbs  from  its  host  these  needed  substances 
also.  Their  absorption  is  accomplished  through  peculiarly 
modified  roots  called  haustoria,  the  wood  or  xylem  elements 
of  which  connect  directly  with  the  wood  elements  of  the 
vascular  bundles  of  the  host.  Through  the  xylem  the  water 
absorbed  from  the  soil  and  the  mineral  salts  dissolved  in 
it  are  transferred  to  the  leaves.  The  mistletoe,  tapping  the 
water-conducting  tissues  of  the  host,  establishes  a  water- 
conducting  system  continuous  with  that  of  the  host,  and  so 
secures  a  supply  of  water  and  mineral  salts  as  constant  and 
as  abundant  as  that  of  the  host.  On  the  other  hand,  the 
chief  paths  of  transfer  for  elaborated  foods,  nitrogenous  as 
well  as  non-nitrogenous,  are  furnished  in  higher  plants  by 
the  phloem  elements  of  the  vascular  bundles,  but  from  these 
the  foods  are  distributed  osmotically  through  parenchyma 
cells  to  the  tissues  needing  them.  The  phloem  elements  of 
the  haustoria  of  Viscum  and  Phoraclendron  are  not  con- 
tinuous with  those  of  the  host.  *  From  this  fact  it  has  been 
inferred  that  mistletoe  does  not  absorb  elaborated  foods  of 
any  sort  from  its  host,  that  it  is,  therefore,  only  a  "water 
parasite.'7  This  inference  is  scarcely  defensible,  though  in  the 
absence  of  direct  evidence  to  the  contrary  the  supposition 
is  justified  that  Viscum  robs  its  host  of  much  less  elaborated 
food  than  those  parasites  which  have  a  direct  phloem,  as 
well  as  xylem,  connection  with  their  hosts. 

*  Peirce.  G.  J.  On  the  structure  of  the  haustoria  of  some  phanerogamic 
parasites.  Annals  of  Botany,  vol.  VII.,  pp.  317,  318,  1893.  Cannon,  W.  A. 
The  anatomy  of  Phoradendron  villosum.  Nutt.  Bull.  Torrey  Bot.  Club, 
vol.  28,  1901. 


88  PLANT  PHYSIOLOGY 

Gaston  Bonnier*  claims  that  Viscum  is  sometimes  directly 
beneficial  to  its  host.  In  summer  the  host  produces  both 
actually  and  proportionally  more  food  than  the  parasite, 
although  the  parasite  is  then  photosynthetically  active.  At 
times  during  the  winter  and  always  during  the  early  spring, 
when  the  host  is  leafless,  the  evergreen  parasite  may  manu- 
facture carbo-hydrates  and  the  host  cannot.  While  the  host 
is  not  able  to  manufacture  food  and  the  parasite  is,  the 
host  is  alleged  to  draw  upon  the  mistletoe  for  freshly  manu- 
factured non-nitrogenous  food.  It  is,  therefore,  claimed  by 
Bonnier  that  the  mistletoe  (  Viscum )  is  parasitic  either  not 
at  all  or  only  very  slightly,  although  obviously  it  obtains 
all  its  water  and  mineral  salts  from  the  host.  But  because 
the  parenchyma  tissues  of  the  parasite  are  continuous 
through  the  haustoria  with  those  of  the  host,  furnishing  the 
paths  of  osmotic  transfer,  the  transfer  taking  place  in  one 
direction  at  one  time  may  be  in  the  opposite  direction  at 
another;  and  so  it  must  be  conceded,  until  proof  to  the 
contrary  is  adduced,  that  the  mistletoe  may  be  more  than 
a  "water  parasite."  Why  should  the  host,  on  the  warm 
days  of  winter  and  in  the  early  spring,  have  any  occasion 
to  draw  food  from  the  mistletoe?  A  healthy  apple-tree,  or 
oak,  or  poplar,  will  normally  lay  by  in  its  own  body  during 
the  summer  enough  elaborated  food  to  start  it  well  in  the 
succeeding  spring.  Upon  this  store  it  will  draw  promptly 
and  satisfyingly  when  the  need  comes.  Is  there  any  less 
food  stored  in  an  apple-tree  upon  which  mistletoe  grows? 
If  so,  is  not  the  mistletoe  the  cause,  and  can  this  lack  ever 
be  wholly  compensated  for?  It  would  appear,  then,  that 
Viscum  is  a  periodic  rather  than  a  partial  parasite,  and 
that  only  further  investigation  can  show  how  nearly  its 
indebtedness  to  its  host  is  annually  balanced. 

Closer  association  with  the  host  und  greater  dependence 
upon  it  are  exhibited  by  the  various  species  of  Cuscuta  or 
dodder,  thread-like  plants  belonging  to  the  Convolvulacese, 

*  Bonnier,  G.  Assimilation  du  Gui  comparee  a  celle  du  pommier.  Actes  du 
Congres  de  1889  d.  1.  Societe  Bot.  de  France :  Bull.  Soc.  Bot.  de  France, 
1890.  Sur  1' assimilation  des  plantes  parasites  a  chlorophylle.  Comptes 
Rendus,  t.  113,  p.  1074-6. 


NUTRITION  89 

but  differing  strikingly  from  the  other  members  of  the 
family  in  habit  and  habits.  They  are  leafless  twiners,  yellow, 
orange,  or  even  sometimes  claret-red,  in  color,  with  very 
little  if  any  chlorophyll.  At  frequent  intervals  their  stems 
and  branches  form  close  coils  around  their  hosts,*  and 
from  the  inner  surfaces  of  these  coils  haustoria  grow  into 
the  tissues  of  the  hosts.  The  haustoria  have  well-developed 
vascular  bundles,  the  xylem  and  phloem  of  which  are  united, 
cell  to  cell,  with  the  xylem  and  phloem  of  the  adjacent  vas- 
cular bundles  in  the  host,  thus  perfectly  connecting,  in  host 
and  parasite,  those  tissues  which  conduct  aqueous  solutions 
of  mineral  salts  and  of  elaborated  foods  respectively,  t 
When  the  dodder  has  fastened  upon  a  suitable  host,  sent 
haustoria  into  it,  and  connected  its  own  vascular  tissues 
with  the  corresponding  ones  of  its  host,  it  draws  food 
in  abundance.  Chlorophyll  develops  only  in  smallest  quan- 
tity in  any  part  of  it.J  The  dodder  absorbs  already  elab- 
orated all  the  sugars  which  it  needs  for  the  construction  of 
cell-wall,  for  the  supply  of  energy  liberated  by  respiration, 
for  the  synthesis  of  amides,  proteids,  etc.  Not  having  chlo- 
rophyll, it  lacks  the  means  by  which  to  secure  the  energy 
needed  for  the  elaboration  of  non-nitrogenous  food,  and  for 
this  food  it  depends  wholly  upon  its  host.  Presumably  it 
takes  its  nitrogenous  food  also  in  the  soluble  forms  elabo- 
rated by  its  host,  though  it  modifies,  assimilates,  and  in- 
corporates this  for  itself.  When  the  dodder,  having  fastened 
upon  an  unsuitable  host,  is  inadequately  fed,  it  may  become 
green  by  the  formation  of  chlorophyll  in  the  chromatophores 
always  present  in  rudimentary  condition  in  its  cortical 
cells.  §  It  can  thus  add  what  it  manufactures  itself  to  the 
insufficient  supply  of  non-nitrogenous  food  which  it  drawrs 
from  its  own  innutritions  host.  The  dodder  make&  food  for 
itself  only  when  it  is  unable  to  secure  enough  ready  made. 

*  See  Chapter  VI.  for  a  discussion  of  the  irritability  of  these  and  other 
plants. 

f  Peirce   G.  J.    L.  c. 

t  Ibid.,  A  contribution  to  the  physiology  of  the  genus  Cuscuta.  Annals 
of  Botany,  vol.  VIII.,  p.  91,  1894. 

§  fbirL,  L.  c..  p.  83. 


90  PLANT  PHYSIOLOGY 

Its  ability  to  develop  chlorophyll  in  times  of  need  suggests 
two  hypotheses  :  that  it  has  only  recently  abandoned  the  in- 
dependent habits  still  followed  by  the  other  members  of  the 
family  Convolvulacese,  and  that  it  has  done  so  in  much 
the  same  way  as  the  mistletoes,  though  the  latter  have 
progressed  by  no  means  so  far  toward  permanent  para- 
sitism. 

It  is  hardly  necessary  to  discuss  whether  the  host  is  bene- 
fited by  the  encircling  dodder,  for  the  dodder  is  an  annual, 
dying  soon  after  ripening  its  fruits,  into  which  it  removes 
the  greater  part  of  the  nutrient  substances  contained  in  and 
composing  its  body.  Thus  it  leaves  little  or  nothing  for  its 
host  to  absorb  and  feed  upon.  When  it  attacks  shrubby 
perennials — e.  g.  willows — its  effects  are  only  impoverishing 
and  debilitating;  when  it  successfully  attacks  annuals,  it 
exhausts  and  kills  them.  European  growers  of  flax  and 
clover  find  the  dodder  one  of  the  most  destructive  enemies 
of  their  crops.  The  dodder  is,  then,  a  permanent  parasite, 
the  parasitism  of  which  is  complete,  however,  only  when  the 
host  can  supply  it  with  all  needed  foods. 

The  most  intimate  associations  between  parasite  and 
host,  and  the  most  complete  dependence  of  a  parasite  upon 
its  host,  are  found  among  the  fungi  and  bacteria  parasitic 
upon  higher  organisms.  In  these  associations  the  parasite 
not  only  sends  root-like  absorbing  organs  into  the  host, 
but  in  many  cases  it  is  very  completely  enclosed  within  the 
host.*  The  parasitic  fungi  and  bacteria,  always  wholly 
devoid  of  chlorophyll,  are  entirely  dependent  upon  their 
hosts  for  both  kinds  of  food,  non-nitrogenous  and  nitro- 
genous. These  foods  they  work  over,  assimilate,  incorpo- 
rate and  consume,  in  their  own  wrays,  but  the  elaborated 
foods  they  must  have.  These,  then,  are  examples  of  per- 
manent and  complete  parasitism.  About  the  nature  of  the 
association  between  green  plants  and  the  fungi  which  cause 
disease  in  them,— e.  g.  grape-mildew,  potato-rot,  wheat-rust, 
etc. — there  cannot  be  the  least  question :  the  host  gains 

*  This  last  is  not  without  parallel  among  phanerogamic  parasites,  for 
Rafflesia,  Brugmnnsia,,  and  our  own  Arceuthobium  (Razoumowskia)  are 
more  or  less  completely  imbedded  in  the  tissues  of  the  host. 


NUTRITION  91 

nothing  from  association   with   the  fungus,    the   parasite 
gains  everything  from  association  with  the  green  plant. 

Only  about  those  remarkable  structures  called  lichens 
is  there  any  difference  of  opinion.  Lichens  are  composed 
of  algae  and  fungi  living  together.  The  algae — green,  blue- 
green,  or  brownish— contain  chlorophyll,  and  by  its  aid  man- 
ufacture non-nitrogenous  foods  from  carbon-dioxide  and 
water.  Nitrates  they  absorb  in  solution.  From  these  and 
the  sugars  they  elaborate  amides  and  proteids  like  other 
independent  plants.  The  fungi,  on  the  contrary,  devoid  of 
chlorophyll,  cannot  elaborate  non-nitrogenous  foods  and 
must  absorb  them  ready  formed.  In  the  lichen  the  fungus  is 
always  closely  applied  to  the  algal  cells,  sends  out  short 
branches  which  clasp  the  algal  cells,  and,  in  a  considerable 
number  of  already  reported  cases,  these  short  branches  send 
still  shorter  haustoria  into  the  algal  cells.*  Whether  only 
closely  applied  to  the  walls,  or  sending  haustoria  into  the 
cells,  the  fungus  filaments  are  so  placed  that  they  can  draw 
food  by  osmosis  from  the  alga.  Because  of  the  small  size  of 
the  alga,  the  always  larger  fungus  cannot  become  entirely 
enclosed  within  it ;  on  the  contrary,  the  fungus  surrounds  the 
alga  with  a  more  or  less  firm  mycelium,  confining  the  alga 
between  the  parts  of  its  body.  The  association  of  fungus 
and  alga,  always  intimate  enough  for  the  fungus  to  supply 
itself  osmotically  with  non-nitrogenous  foods  elaborated  by 
the  alga,  is  in  many  cases  so  exhausting  to  the  alga  that 
many  of  its  cells  become  entirely  emptied.  In  spite  of  this 
evidence  of  the  complete  parasitism  of  the  fungus,  some 
botanists  claim  that  the  alga  benefits  also.  It  is  alleged 
that  the  carbon-dioxide  exhaled  by  the  fungus,  the  mineral 
salts  dissolved  and  held  in  solution,  the  protection  against 
too  rapid  drying,  too  intense  illumination,  and  too  sudden 
changes  of  temperature,  are  of  sufficient  value  to  the 
alga  to  compensate  it  for  the  food  taken  from  it  and  for 
the  deformities  and  limitations  in  its  growth.  It  may  be 

*  Peirce,  G.  J.  The  nature  of  the  association  of  alga  and  fungus  in 
lichens.  Proc.  Cal.  Acad.  Sci.,  Series  III.,  Botany,  vol.  I.,  1899.  The  rela- 
tion of  fungus  and  alga  in  lichens.  American  Naturalist,  vol.  XXXTV., 
1900. 


92  PLANT  PHYSIOLOGY 

true  that  all  these  benefits  do  accrue  to  the  alga — though 
this  is  very  far  from  being  demonstrated — but  even  if  this  be 
true,  are  these  benefits  needed,  are  they  not  superfluous? 
Man's  association  with  his  domesticated  animals  is  bene- 
ficial to  them,  but  are  these  animals  really  any  better  off 
than  by  themselves  in  nature?  Even  if  man's  association 
with  them  is  beneficial  for  the  time,  in  the  end  it  is  fatal  or, 
at  least,  onerous.  The  absolute  dependence  of  the  fungus 
component  of  the  lichen  upon  some  green  plant  for  food, 
and  the  damage  and  death  to  the  alga  wrought  by  the 
fungus,  furnish  the  strongest  evidence  that  the  association 
is  not  equally  beneficial  to  the  two  members,  mutually  bene- 
ficial though  it  is  sometimes  claimed  to  be. 

In  these  cases,  we  have  examples  of  the  stages  through 
which  parasitism  has  advanced — first,  green  plants,  incom- 
pletely and  only  periodically  parasitic;  second,  plants  nor- 
mally not  green,  permanently  parasitic,  and  completely 
parasitic  when  the  host  is  suitable;  third,  plants  never 
green,  permanently  and  completely  parasitic. 

The  bacteria  living  in  the  bodies  of  animals,  either  inter- 
cellularly  or  intracellularly,  absorbing  already  elaborated 
foods,  utilizing  these  by  processes  fundamentally  like  those 
already  discussed,  present  no  new  physiological  principles, 
and  hence  those  who  would  know  more  of  this,  as  of  the 
other  special  groups  of  organisms  which  we  have  just  been 
discussing,  must  turn  to  the  treatises  devoted  to  them. 

OTHER  ELEMENTS  ESSENTIAL  TO  PLANTS 

The  physiological  chemistry  of  the  other  elements  con- 
cerned in  the  nutrition  of  plants  is  still  so  vague  that  little 
can  be  said  about  them  till  further  investigations  give 
definite  facts  to  deal  with.  These  elements  are  obtained  in 
analyses  of  plants  in  the  form  of  incombustible  compounds 
forming  the  ash  resulting  from  combustion.  For  this  reason 
they  are  collectively  termed  ash-constituents,  to  distinguish 
them  from  hydrogen,  oxygen,  carbon,  and  nitrogen,  which 
go  off  in  drying  and  burning.  Besides  the  salts  containing 
the  elements  absolutely  essential  to  the  normal  nutrition  of 
the  plant,  others  are  invariably  present  in  the  ash  because 


NUTRITION  93 

they  are  present  in  soluble  form  in  the  soil.  Though  some 
few  of  these  may  be  useful,  they  are  not  necessarily  es- 
sential. 

Chemical  analysis  reveals  whatever  is  present  in  the  plant- 
body,  but  neither  indicates  the  compound  in  which  an  ele- 
ment exists  in  the  living  body,  nor  enables  one  to  distin- 
guish between  necessary  substances  and  those  present  in  the 
plant  simply  because  they  are  present  in  the  media  in  which 
it  lives— in  soil,  air,  and  water.  Only  by  the  culture  of 
plants  in  media  of  known  composition  can  the  essential 
elements  and  compounds  be  distinguished  from  the  non- 
essential,  the  useful  but  not  essential  from  the  absolutely 
useless  and  the  absolutely  necessary.  Analysis  shows  that 
ordinarily  1.5-5%  of  the  dry  weight  of  plants  is  furnished  by 
the  ash  constituents  of  all  sorts,  though  in  some  cases 
10-30%  is  ash.  Analysis  alone  cannot  account  for  this  dis- 
crepancy. Culture  in  media  of  known  composition  shows 
that  the  greater  amount  of  ash  is  due  to  peculiarities  of 
the  soil  or  to  peculiarities  of  certain  species  or  even  families 
of  plants.  For  example,  18-23%  of  the  ash  of  Indian  Corn 
is  silicic  dioxide,  useful  to  the  plant  in  hardening  its  outer 
surfaces  and  making  projecting  parts  and  the  edges  of 
leaves  harsh  and  cutting,  but  not  essential  to  its  stiffness, 
growth,  and  perfect  maturity.  *  Diatoms  and  the  scouring- 
rushes  (Equisetum)  are  much  richer  in  silica  than  the 
grasses,  but  it  is  not  yet  proved  that  it  is  indispensable 
even  for  them. 

Analysis  reveals  the  presence  of  sodium  and  chlorine,  as 
common  salt,  in  all  plants.  Because  culture  without  these 
elements  is  so  difficult  that  it  is  doubtful  whether  it  has 
ever  been  accomplished,  no  one  can  say  whether  they  are 
absolutely  essential  or  not.  Experiment  has  already  con- 
clusively shown,  however,  that  they  are  needed  only  in  the 
smallest  possible  quantities  if  at  all,  though  in  certain  cases 
larger  quantities  may  act  as  favorable  stimulants.  This 
last  is  especially  evident  in  the  bacteria,  the  growth  of 
which  in  the  artificial  culture  media  and  under  the  unnatural 

*  Quoted  by  Pfeffer  (Pflanzenphysiologie,  Bd.  I.,  p.  429,  Engl.  tranel. 
p.  435)  from  Sachs  (Flora,  p.  52,  1862). 


94  PLANT  PHYSIOLOGY 

Conditions  of  the  laboratory  seems  to  be  facilitated  by  the 
addition  of  a  small  amount  of  common  salt  to  the  culture. 
The  ash  of  strand  and  marine  plants  contains  a  larger  per- 
centage of  sodic  chloride  than  that  of  inland  plants.  This 
is  due  simply  to  the  presence  of  so  much  salt  where  they 
grow.  Strand  plants  do  not  need  salt,  as  is  proved  by 
cultures.*  Inland  plants  are  unfavorably  influenced  by  a 
percentage  of  salt,  in  the  soil  or  in  water,  which  strand 
plants  bear  without  injury.  These  last  have  succeeded  in 
becoming  adapted  to  conditions  which  preclude  or  minimize 
competition  from  more  sensitive  forms.  The  adaptations 
are  discussed  in  the  rapidly  increasing  literature  on  the 
ecology  of  the  so-called  halophytes.f  Although  the  common 
salt  in  sea  water  is  needed  as  food  only  in  the  smallest 
quantities  if  at  all  by  the  marine  algge,  they  will  bear  only 
the  most  gradual  transfer  to  fresh  water.  This  is  probably 
due,  however,  to  the  greater  density  of  the  sea  water,  and 
this  depends  upon  the  other  salts  dissolved  in  it  as  well  as 
upon  this  single  one. 

An  interesting  experimental  study  of  strand  and  other 
plants  with  relation  to  common  salt  and  sea  water  has  re- 
cently been  made  by  Coupin.J;  He  finds  that  1.5%  of  com- 
mon salt  in  soil  or  in  water  is  poisonous  to  plants  which 
do  not  naturally  grow  on  the  sea-shore.  Since  sea 
water  contains  about  2.5%  of  common  salt  and  the  soils 
bathed  by  the  sea  contain  still  more  than  this  proportion, 
we  can  readily  understand  the  sharp  line  which  separates 
the  marine  and  strand  floras  from  those  of  the  interior. 
€oupin  attributes  the  poisonous  property  of  sea  water  for 
inland  plants  mainly  to  its  content  of  common  salt,  for  the 
two  salts  next  to  this  in  abundance,  magnesium  sulphate 
and  chloride,  are  present  in  quantities  which  he  says  are 
below  the  toxic  proportions.  Magnesic  sulphate  is  poison- 

*  See  Nobbe  in  Versuchsetationen,  Bd.  13,  1870,  the  literature  there 
cited,  and  Pfeffer,  Pflanzenphysiologie,  I.,  p.  424,  etc.,  Engl.  transl.  I. 
pp.  429,  30,  etc. 

f  For  example,  Schimper,  A.  F.  W.  Pflanzengeographie  auf  physiolo- 
gischer  Grundlage,  Jena,  1898. 

t  Coupin,  H.  Sur  la  toxicite  du  chlomre  de  sodium  et  de  1'eau  de  mer 
a  1'egard  dee  vegetaux.  Rev.  gener.  de  Botanique,  T.  X.,  1898. 


NUTRITION  95 

ous  at  a  concentration  of  1%,  magnesic  chloride  at  0.8%, 
but  they  occur  in  sea  water  only  to  the  extent  of  0.75% 
and  0.6%  respectively.  For  strand  plants  the  propor- 
tions of  common  salt  are  very  different,  as  these  figures 
indicate : 

Fatal.          Injurious.       Harmless. 

Beta  maritima  4%  3%  2.6% 
Atriplex  hastata 

var.  maritima  5%  4%  3.5% 

Cakile  maritima  4%  3%  2.8% 

For  these  three  plants  3%  of  magnesic  sulphate,  2.5%  of 
magnesic  chloride,  are  poisonous.  From  these  figures  it  is 
obvious  that  strand  plants  are  very  accurately  adapted  to 
the  amount  of  common  salt  to  which  they  are  exposed,  and 
that  they  can  withstand  much  more  of  the  other  salts  than 
they  are  ever  exposed  to.* 

The  soluble  compounds  of  zinc  are  poisonous  to  all  plants. 
In  quantities  not  excessive  when  first  encountered,  or  in 
amounts  to  which  the  special  plants  have  become  accus- 
tomed by  long  habit,  zinc  salts  seem  either  to  be  ineffective 
or  else  to  act  as  stimulants  to  more  active  growth  and 
life.t  It  is  claimed  that  there  is  a  flora  characteristic  of 
soils  rich  in  zinc.  This  is  an  exaggeration,  but  it  cannot  be 
denied  that  certain  plants  are  found  on  zinc  soils  and  not 
elsewhere  (e.  g.  Viola  calaminaria  and  Thlaspi  calami- 
narium ) .  These,  however,  are  varieties  of  other  species  (  viz. 
of  V.  lutea  and  T.  alpestre),  the  variation  being  induced  by 
the  poison,  t 

Aluminum  salts,  though  of  very  general  occurrence,   are 

*  Schimper  claims  (L.  c.  pp.  98  —  102)  that  plants  living  in  soils  rich  in 
freely  soluble  salts  like  sodic  chloride,  saltpeter,  etc.,  present  structurarand 
other  characters  almost  identical  with  those  of  desert  plants.  This  he  re- 
gards as  evidence  that  halophytes  as  well  as  xerophytes  seek  to  reduce 
transpiration,  the  latter  because  of  the  scarcity  of  water,  the  former  be- 
cause of  the  presence  in  it  of  poisonous  compounds. 

\  Richards,  H.  M.  Beeinflussung  des  Wachsthums  einiger  Pilze  durch 
chemische  Reize.  Jahrb.  f.  w.  Bot.,  Bd.  30,  1897. 

\  Schimper,  A.  F.  W.  Pflanzengeographie  auf  physiologischer  Grundlage, 
Jena,  1898. 


96  PLANT  PHYSIOLOGY 

found  in  quantity  only  in  the  lycopods.*  It  is  doubtful 
whether  aluminum  is  necessary  or  even  useful  for  these 
plants. 

The  essential  soil  ( ash )  constituents,  as  shown  by  water- 
culture,  are  salts  of  phosphorus;  sulphur,  potassium,  cal- 
cium, magnesium,  and  iron.  Calcium  seems  to  be  un- 
necessary for  fungi,  though  indispensable  for  higher 
plants. 

PHOSPHORUS  is  a  constituent  element  of  protoplasmic 
matters.  In  the  nucleins  it  may  amount  to  6%.  According 
to  Wolff's  analyses, t  phosphoric  oxide  (P205)  constitutes 
about  one-third  of  the  ash  obtained  from  embryonic  tissue. 
This  tissue  is  rich  in  protoplasmic  matters.  In  older  tissues 
containing  a  smaller  proportion  of  protoplasmic  matters, 
and  in  dead  and  emptied  cells,  the  amount  of  phosphorus 
compounds  is  much  less.  In  the  total  dry  weight  of  a 
plant,  the  amount  of  phosphorus,  calculated  as  phosphoric 
acid,  is  slight.  This  is  shown  by  the  following  figures — J 

in    lupine  seeds  1.63% 

straw  0.30% 

"    potato  tubers  0.63% 

"    wood  of  trees  0.05% 

Though  the  percentage  of  phosphorus  in  the  body  of  an  or- 
ganism indicates  the  degree  to  which  it  is  used,  it  by  no 
means  indicates  the  degree  in  which  it  is  needed.  Without 
phosphorus,-  protoplasm  could  not  exist. 

The  source  of  phosphorus  for  the  majority  of  plants  is 
the  phosphates  in  soil  and  water.  Other  plants  under 

*  Pfeffer,  W.  Pflanzenphysiologie,  I.,  p.  432,  Engl.  transl.  I.,  p.  437. 
L.  Chamsecyparissus  and  //.  Alpinum  have  22-27%  aluminum  in  the  ash, 
while  L.  phlegmfiria,  Selaginella,  etc.,  contain  only  traces.  Yoshida,  H. 
On  aluminum  in  the  ash  of  flowering  plants.  Journ.  Coll.  Science,  Im- 
perial University,  Tokio,  1887.  In  analyses  of  rice,  wheat,  oats,  beans, 
etc.  the  Al.  varies  from  0.05%  to  0.27%  of  the  ash. 

f  Quoted  from  Versuchsstationen,  Bd.  30,  by  Pfeffer  in  his  Pflanzen- 
physiologie, L,  p.  407,  Eng.  transl.  I.,  p.  414. 

t  Frank,  A.  B.  Lehrbuch  der  Botanik,  Bd.  I.,  p.  587.  Also  in  reports 
of  State  Agricultural  Experiment  Stations,  etc.,  similar  figures  may  be 
found. 


NUTRITION  97 

natural  conditions  may  at  all  times  supply  themselves  with 
phosphorus  from  its  organic  compounds  in  humus,  in  dead 
bodies  of  plants  and  animals,  and  in  living  bodies.  The 
phosphates  are  less  freely  soluble  in  water  than  the  salts  of 
the  other  necessary  elements ;  but  since  plants  demand  only 
small  quantities,  and  since  the  carbon-dioxide  and  possibly 
also  other  acid  secretions  of  their  roots  dissolve  them  more 
rapidly  than  does  pure  water,  plants  secure  under  ordinary 
conditions  in  nature  all  the  phosphates  needed.  Manuring 
with  phosphates  has  to  be  resorted  to  only  wiien  the  soil 
has  become  impoverished  by  the  removal  of  the  crops  culti- 
vated upon  it. 

In  the  plant  phosphorus  occurs  not  only  as  a  constituent 
element  of  living  protoplasm,  but  also  in  those  already 
highly  elaborated  substances  to  be  used  in  the  construction 
of  protoplasm,  and  in  the  simpler  compounds  formed  by  the 
breaking  up  of  living  or  lifeless  protoplasmic  matter.  The 
compounds  of  phosphorus  occur,  therefore,  in  the  plant  as 
solids— as  parts  of  its  structure  and  as  stored  material; 
also  in  solution— as  constructive  material  and  as  the  pro- 
ducts of  destructive  metabolism. 

SULPHUR,  also  a  constituent  element  of  protoplasm  and 
therefore  always  present  in  the  ash  of  plants,  is  found  in 
even  smaller  proportions  in  the  plant-body  than  phos- 
phorus, as  the  following  figures  show — * 

in  lupine  seeds  0.36% 

"   potato  leaves  0.43% 

tubers  0.24% 

"    wood  of  trees  0.025% 

The  source  of  sulphur  for  most  plants  is  the  various  salts 
of  sulphuric  acid  commonly  found  in  the  soil  and  dissolved 
in  ordinary  waters.  Dependent  plants— the  so-called  para- 
sites and  saprophytes— may  perhaps  obtain  some  sulphur  in 
the  form  of  organic  compounds.  The  moulds  can  use  salts 
of  sulphurous  acid,  if  present  in  sufficiently  dilute  solution, f 
although  they  are  nearly  as  poisonous  to  all  higher  plants 

*  Frank,  A.  B.  Lehrbuch,  I.,  p.  586. 

f  Quoted  by  Pfeffer  from  Nageli,  Bot.  Mittheilungen,  Bd.  3,  1881. 

7 


98  PLANT  PHYSIOLOGY 

as  sulphurous  oxide  and  dioxide,  gases  poured  forth  in  con- 
siderable quantities  from  chimneys  in  which  inferior  sorts  of 
coal  are  burned.*  A  few  species  of  bacteria  use  as  food- 
material,  as  well  as  their  source  of  energy  by  respiration 
(see  page  20),  the  sulphuretted  hydrogen  and  metallic 
sulphur  occurring  in  considerable  quantities  in  mineral 
springs  and  set  free  in  decompositions  taking  place  in  the 
ooze  under  bodies  of  water. 

Sulphur  occurs  in  plants  in  the  elementary  condition  only 
in  well-fed  sulphur  bacteria.  In  these,  as  in  all  other  plants, 
it  occurs  also  as  a  constituent  of  protoplasm.  A  few  plants, 
notably  the  Cruciferze,  contain  sulphur  in  mustard  oil,  and 
it  is  a  constituent  of  the  garlic  oil  in  the  various  species  of 
Allium. 

Nothing  whatever  is  known  of  the  stages  through  which 
the  amides  are  elaborated  into  protoplasmic  matters  by  the 
addition  of  sulphur  and  phosphorus. 

POTASSIUM,  although  not  a  constituent  element  of  proto- 
plasmic substances,  is  an  indispensable  food-constituent. 
The  compounds  of  sodium,  frequently  much  more  abundant, 
fail  to  serve  as  complete  substitutes,  although  in  the  pres- 
ence of  an  abundance  of  sodium  salts  plants  demand  less 
potassium  than  otherwise.  In  the  total  dry  weight  of  plants 
potassium  occurs  in  much  larger  amount  than  sulphur  and 
phosphorus— t 

in  potato  tubers  2.27% 

plants  2.53$ 

"   tobacco  leaves  4.99% 

"   young  red  clover  plants  3.59% 

"    lupine  seeds  1.31$ 

"    wood  of  trees  G.05%-0.15% 

Potassium  salts  are  most  abundant  in  young  and  growing 
parts,  least  abundant  in  those  which  have  ceased  to  grow 
or  to  be  otherwise  active,  and  from  which  the  potassium 

*  The  striking  absence  from  certain  cities  in  which  one  might  otherwise 
expect  to  find  them,  of  lichens  and  other  plants  especially  sensitive  to 
gaseous  poisons,  may  be  attributed  to  the  poor  coal. 

t  Frank.    Lehrbuch,  I.,  p.  589. 


NUTRITION  99 

has  been  withdrawn.  From  this  it  appears  that  potassium 
compounds  are  intimately  concerned  in  the  construction  of 
protoplasmic  matters.  What  compounds  these  are  is  not 
now  known,  though  it  is  evident  that  potassium  may  be  a 
component  of  reserve  foods.  Nobbe's  hypothesis*  that  po- 
tassium salts  are  directly  concerned  in  the  translocation  of 
the  starch  formed  in  chlorophyll-containing  cells  is  interest- 
ing, because  it  suggests  one  of  their  possible  uses  in  the 
plant,  but  that  this  is  their  main  function  follows  neither 
from  his  experiments  nor  his  arguments. 

According  to  Copeland,t  the  potassium  salts  are4 'an  im- 
portant part  of  the  osmotically  active  material  which  keeps 
the  cell  and  plant  turgid,"  and  "there  is  no  experimental 
ground  for  attaching  this  significance  to  any  other  con- 
stituent of  the  mineral  food." 

Potassium  salts  are  never  very  abundant  in  soils.  They 
occur  as  sulphate,  phosphate,  and  chloride,  freely  soluble, 
and  hence  easily  washed  away  as  well  as  absorbed.  It  is 
necessary  frequently  to  manure  soils  with  potassium  salts 
when  tobacco  and  other  crops  demanding  considerable 
amounts  of  potassium  are  cultivated  year  after  year  on 
the  same  soil. 

CALCIUM  is  neither  a  constituent  of  protoplasm  nor  neces- 
sary to  all  plants.  For  the  fungi,  and  perhaps  for  certain 
algae,  I  it  is  entirely  unnecessary,  magnesium  successfully 
taking  the  place  of  calcium,  besides  performing  its  own  part 
in  nutrition.  All  other  plants  demand  both  calcium  and 
magnesium.  Unlike  the  elements  already  considered,  the 
salts  of  calcium  are  found  in  tissues  which  have  attained 
their  full  growth  and  in  which  work  of  another  kind  is 
especially  going  on,  namely,  in  the  chlorophyll-containing 
food-making  cells  of  the  leaves  and  cortex.  In  organs  in 
which  elaborated  foods  are  stored,  and  in  such  dead  parts 

*  Nobbe,  in  Landwirtschaftliche  Versuchsstationen,  Bd.  13,  1870. 

t  Copeland,  E.  B.  Relation  of  nutrient  salts  to  turgor.  Botanical 
Gazette,  Vol.  24,  1897. 

t  Molisch,  H.  In  Botanisches  Centralblatt,  Bd.  60,  1894,  and  Sitzungs- 
ber.  d.  Wiener  Akademie,  Bd.  103,  104,  105,  Abth.  I.,  1894,  1895,  1896. 
Benecke,  W-,  in  Botanisches  Centralblatt,  Bd.  60,  1894.  Jahrbuch  f. 
wiss.  Botanik,  Bd.  28,  189,  and  Botanische  Zeitung,  1896. 


100  PLANT  PHYSIOLOGY 

as  the  wood,  calcium  salts  are  less  abundant.    These  facts 
are  indicated  by  the  following  table — * 

Potato  leaves  have  2.90%  of  dry  weight  as  calcium  salt. 

tubers  0.10% 

Pea  straw  1.88% 

"    seeds  0.13% 

Tobacco  leaves  6.18% 

Hop  "  7.38% 

Wood  of  trees  0.02-0.10% 

From  such  figures,  the  result  of  gross  analyses  confirmed 
by  microchemical  tests,  it  has  been  inferred  that  calcium 
is  concerned  in  the  formation  of  cell-wall  and  in  neutralizing 
the  oxalic  acid  set  free  in  various  chemical  changes  ( notably 
respiration)  taking  place  in  plant  cells.  Proof  that  these 
inferences  are  correct  is  still  lacking,  however. 

Loew's  hypothesis!  that  the  framework  of  nucleus  and 
plastids  is  a  double  organic  salt  of  calcium  and  magnesium, 
or  a  complex  union  of  calcium  and  magnesium  compounds, 
is  a  hypothesis  only,  and  hence  need  only  be  mentioned. 

The  value  of  calcium  to  higher  plants  is  beyond  question, 
but  how  it  is  used  is  still  unknown.  It  is  found  in  plants 
usually  as  the  oxalate  (crystallized  out  as  needles  or  as 
polyhedra  of  more  symmetrical  dimensions),  as  carbonate 
( deposited  in  the  peculiar  outgrowths  of  cell-wall  known  as 
cystoliths),  much  less  frequently  as  sulphate  and  phos- 
phate. Calcium  salts  are  abundant  enough  in  the  soil  to  be 
taken  up  by  all  plants,  and  although  the  sulphate  and 
phosphate  are  only  slightly  soluble,  the  large  volumes  of 
water  absorbed  will  still  carry  adequate  amounts,  even  from 
soils  which  contain  little  or  no  other  calcium  compounds, 
to  the  cells  using  them. 

MAGNESIUM,  absolutely  indispensable  to  all  plants, $  in 
spite  of  the  assertions  of  earlier  authors  to  the  contrary,  § 

*  Frank's  Lehrbuch,  I.,  p.  590. 

t  Loew,  0.  Uber  die  physiologischen  Functionen  der  Calcium-und  Mag- 
nesiumsalze  im  Pflanzenorganismus.  Flora,  1892.  Uber  das  Mineralstoff- 
bediir  f.  niss  von  Pflanzenzellen.  Bot.  Centralblatt,  Bd.  63,  1895. 

t  Molisch,  H.    L.  c.  under  calcium.    Benecke,  W.    L.  c.  under  calcium. 

§  Nageli.    Botanische  Mittheilungen,  Bd.  3,  1881  and  others. 


NUTRITION  101 

is  even  less  understood  in  its  physiological  relations  than 
calcium.  As  phosphate  it  may  stand  in  some  relation  to 
the  formation  of  protoplasmic  matters,  although  it  is  not  a 
constituent  element  of  protoplasm.  Like  calcium,  it  occurs 
as  a  constituent  of  some  of  the  reserve  foods  stored  hi  such 
oily  seeds  as  Castor  Bean  (Ricinus)  and  Brazil  Nut  (Ber- 
tholetia ) ;  but  even  here,  hi  these  complex  compounds,  it  is 
not  clear  whether  it  is  the  magnesium  ( or  calcium )  itself,  or 
the  acid  radicle  of  the  salt,  which  is  valuable. 

IRON.  Although  this  element  is  not  known  to  be  a  con- 
stituent of  protoplasm  or  of  any  compound  incorporated 
into  the  living  protoplasm,  it  is  indispensable  to  all  plants. 
The  minimum  needed  is,  however,  smaller  than  that  of  any 
other  element.  If  more  than  the  minimum  amount  of  iron 
is  supplied,  growth  seems  to  be  proportionally  stimulated.* 
Unless  green  plants  receive  enough  iron  they  will  not  be 
able  to  form  chlorophyll,  although  iron  is  not  a  constituent 
element  of  any  chlorophyll  pigment.  Plants  remaining 
white  from  lack  of  iron  are  said  to  be  chlorotic.  Chlorosis 
may  be  regarded  either  as  the  evidence  of  an  abnormal 
state  of  health  or  as  the  disease  itself.  The  former  seems 
by  far  the  more  probable. 

The  plant  ordinarily  obtains  all  the  iron  it  needs  from 
the  salts  of  iron  dissolved  in  all  natural  waters.  Sometimes, 
however,  chlorosis  occurs  in  spite  of  the  abundance  of  iron 
in  the  soil.  When  a  shoot  grows  so  rapidly  that  iron  salts 
do  not  reach  the  developing  parts  rapidly  enough,  the  new 
leaves  will  be  white  instead  of  green. 

Some  of  each  of  the  necessary  elements  found  in  the  ash 
of  plants  is  necessary  to  the  normal  development  of  the 
plant.  The  plant  will  develop  normally  only  when  it  can 
obtain  an  amount  of  mineral  matter  in  solution  more  than 
equal  to  the  sum  of  the  minimal  amounts  of  each  of  the 
ash  constituents.  In  virgin  soil,  and  in  the  natural  forest, 
the  soil  waters  will  contain  all  the  necessary  mineral  salts 

*  Richards,  H.  M.  Die  Beeinflussung  dee  Wachsthums  einiger  Pilze  diirch 
chemische  Reize.  Jahrb.  f.  wise.  Botanik,  Bd.  30,  1897.  The  effect  of 
chemical  irritation  on  the  economic  coefficient  of  sugar.  Bull.  Torrey  Bot. 
Club,  vol.  26.  1899. 


102  PLANT  PHYSIOLOGY 

dissolved  in  sufficient  quantities  for  normal  growth.  Where 
man  interferes  by  defective  cultivation  or  mistaken  selection 
of  crops,  the  soil  must  be  artificially  enriched.  In  nature 
the  constant  supply  of  adequate  amounts  of  all  the  food- 
materials  is  secured  by  the  action  of  those  forces  and  or- 
ganisms which  break  down  the  inorganic  and  organic  mat- 
ters on  and  in  the  upper  layers  of  rock  and  soil. 


CHAPTER  IV 

ABSORPTION  AND  MOVEMENT  OF  WATER     FOOD  DISTRIBUTION 

IN  the  preceding  chapter  we  -have  discussed  the  elements 
and  their  compounds,  which  constitute  the  food-materials  of 
plants,  and  we  have  gained  some  idea  as  to  when,  by  what 
means,  and  through  what  stages  these  are  elaborated  into 
foods.  How  foods  are  assimilated— rendered  like  the  living 
protoplasm  which  they  are  to  nourish— is  not  now  evident. 
We  have  finally  seen  that  these  assimilated  foods  may  be 
incorporated  into  and  made  a  part  of  the  living  protoplasm, 
but  how  this  is  accomplished  remains  one  of  the  wholly 
unsolved  problems  of  physiology. 

We  must  now  consider  how  the  plant  absorbs  its  food- 
materials  and  transfers  from  part  to  part  the  foods  which 
it  elaborates  from  them.  These  processes  underlie  and  at 
the  same  time  form  an  essential  part  of  nutrition,  but 
since  other  substances  than  those  needed  and  used  as  food- 
materials  and  foods  are  concerned,  we  may  well  consider 
this  subject  by  itself. 

With  the  exception  of  water,  which  is  both  a  food-material 
and  the  vehicle  of  nutrient  substances,  the  food-materials  of 
animals  and  plants  are  of  two  sorts — either  gases  or  solids. 
The  latter  are  available  only  in  solution,  entering  the  body 
and  passing  from  part  to  part  only  when  dissolved  in 
water.  *  The  absorption  and  transfer  from  part  to  part  in 
the  plant-body  of  the  gaseous  food-materials— carbon-diox- 

*  The  apparent  contradiction  to  this  statement  offered  by  the  Myxomy- 
cetes,  Amoeba,  etc.— naked  masses  of  protoplasm  not  enclosed  by  cell-wall— 
which  may  in  their  movements  surround  solid  particles,  both  innutritions 
and  nutritious,  is  not  a  real  one.  Only  those  particles  which  are  soluble 
and  dissolved  are  absorbed  by  the  protoplasm  and  really  enter  it;  the 
others  are  left  unaffected  by  it  although  they  may  remain  enclosed  for  a 
time. 


104  PLANT  PHYSIOLOGY 

ide,  oxygen,  and  free  nitrogen  ( when  the  last  is  used  at  all ) 
— take  place  in  accordance  with  the  simple  and  generally 
known  laws  governing  the  diffusion  of  gases.  We  must, 
however,  bear  distinctly  in  mind  that  these  gases  are  in 
two  states  in  the  bodies  of  all  higher  and  of  many  lower 
plants.  Through  the  stomata  gases  pass  in  the  gaseous 
condition  into  or  from  the  intercellular  spaces,,  the  air  pas- 
sages, etc.  This  movement  through  the  stomata  and  in  the 
intercellular  spaces  is  strictly  by  diffusion,  except  where 
affected  by  mechanical  forces  such  as  compress  or  expand 
the  air-spaces,  etc.,  etc.  But  when  gases  pass  through  the 
cell-wall,  into  or  out  from  a  cell,  their  molecules  mix  with 
the  water-molecules  in  cell-wall,  protoplasm,  and  vacuoles, 
becoming  dissolved  in  the  water  which  the  living  body  con- 
tains as  an  essential  part  of  its  structure.  The  movements 
of  gases  into  and  out  from  living  cells,  and  from  cell  to  cell, 
are  therefore  the  movements  of  solutes  (dissolved  sub- 
stances ) .  The  absorption  of  solutions  into  living  cells,  and 
their  transfer  from  cell  to  cell,  take  place  in  accordance 
with  the  laws  governing  the  diffusion  of  liquids.  We  can, 
therefore,  study  the  movements  of  solids  and  of  gases  at  the 
same  time,  for,  so  far  as  living  cells  are  concerned,  these 
two  classes  of  substances  behave  alike.  So  far  as  the  supply 
of  solids  and  of  gases  to  the  living  cells  is  concerned,  we 
have  to  deal  with  different  phenomena,  and  these  we  must 
study  separately. 

The  water  in  the  soil,  and  consequently  all  flowing  water 
and  that  in  pools,  ponds,  lakes,  and  in  the  sea,  is  a  dilute 
solution  of  nutrient  and  other  soluble  substances  from  the 
air,  from  the  mineral  matters  of  the  soil,  and  from  the 
mixture  of  organic  and  inorganic  matters  collectively  termed 
humus,  which  is  found  in  all  but  the  most  sterile  soil.  The 
water  of  streams,  ponds,  lakes,  and  of  the  sea,  is  in  the 
hydrostatic  or  massive  state.  After  heavy  rain,  flood,  or 
the  melting  of  snow  and  ice,  water  is  in  the  hydrostatic 
state  in  the  soil  also.  Water  in  this  condition  can  be 
drained  off,  but  much  will  remain  in  other  conditions,  the 
amount  depending  upon  the  character  of  the  soil.  Soil 
which  has  been  thoroughly  drained,  but  not  dried,  will  feel 


ABSORPTION  AND  MOVEMENT  OF  WATER  105 

damp  to  the  touch  because  of  the  considerable  amount  of 
water  held  in  the  capillary  state  between  the  soil  particles. 
This  can  be  removed  by  applying  material  possessing 
stronger  capillary  attraction,  for  instance,  blotting  or  fil- 
ter paper,  which  will  quickly  become  damp  by  withdrawing 
water  from  the  soil  capillaries.*  After  soil  has  been  dried 
as  thoroughly  as  possible  by  removing  the  capillary  water 
through  stronger  capillary  attraction,  water  will  still  be 
retained  in  the  hygroscopic  state,  held  on  the  surfaces  of  the 
soil  particles  themselves.  To  overcome  the  attraction  of  the 
soil  particles  and  to  remove  the  last  traces  of  water  much 
greater  force  must  be  employed.  In  order  to  make  soil 
absolutely  dry  it  must  be  taken  away  from  its  natural 
position  and  exposed  to  some  powerful  dehydrating  influ- 
ence, e.  g.  concentrated  sulphuric  or  glacial  phosphoric  acid- 
in  a  desiccator,  or,  more  simply,  to  heat  in  an  open  vessel. 
Another  means  of  demonstrating  the  very  considerable  force 
by  which  soil  particles  attract  and  hold  water  and  the  sub- 
stances in  solution  in  it,  is  by  filtering  a  dilute  solution  of 
some  convenient  copper  salt  or  of  Fuchsin  through  soil. 
The  last  trace  of  copper  or  of  color  will  be  removed  from 
the  solution,  and  the  filtrate  may  be  successfully  used  for 
the  culture  of  plants,  though  the  original  copper  solution 
would  have  been  poisonous. 

The  amounts  of  water  occurring  in  the  hydrostatic,  capil- 
lary, and  hygroscopic  states  in  soils  will  vary  with  their 
composition,  with  the  fineness  of  the  particles,  and  with  their 
compactness  on  the  surf  ace.  f  It  is  essential  that  plants 
growing  in  coarse  soils  which  drain  rapidly,  and  in  re- 
gions where  no  rain  falls  during  the  growing  season, 
should  be  able  to  supply  themselves  with  water  even  after 
the  hydrostatic  and  capillary  waters  have  been  entirely  re- 
moved from  those  layers  of  the  soil  traversed  by  the  roots. 
To  accomplish  this  it  is  absolutely  necessary  that  the  plant 
should  be  able  to  exert  sufficient  force  to  draw  into  its  own 

*  It  must,  of  course,  be  noted  that  the  cellulose  walls  of  the  capillaries 
in  filter  paper  also  imbibe  water. 

t  See  Year  Book  U.  S.  Dep't  Agriculture,  1897,  p.  129,  report  by  M. 
Whitney,  and  also  other  papers  on  Soils  published  by  U.  S.  Dep't  Agriculture. 


106  PLANT  PHYSIOLOGY 

body  the  water  so  strongly  held  on  the  surfaces  of  the  soiLpar- 
ticles.  In  California,  the  only  water  upon  which  most  plants 
not  subjected  to  irrigation  can  draw  during  the  greater  part 
of  the  dry  season  is  that  held  hygroscopically,  and  that  plants 
grow  at  all  or  even  survive  during  the  dry  season  is  positive 
evidence  that  they  do  exert  such  an  attractive  force.  What 
are  the  means  at  hand?  To  answer  this  question  we  must 
consider  the  physical  properties  of  vegetable  cells. 

The  typical  vegetable  cell — an  alga,  a  root-hair,  a  paren- 
chyma cell — is  bounded  by  a  thin  cellulose  membrane  per- 
meated with  water  holding  in  solution  a  variety  of  mineral 
and  other  substances.  This  membrane  is  firm,  strong, 
elastic,  and  is  not  only  permeated  with  water,  7.  e.  has 
molecules  of  water  between  its  molecules  or  groups  of  mole- 
cules, but  also  permits  the  movement  of  molecules  of  water 
and  of  substances  in  solution  in  water  in  and  through  itself. 
The  movement  of  water  and  of  aqueous  solutions  through 
a  membrane  is  known  as  osmosis.  Lining  the  cellulose 
wall  is  the  layer  of  living  protoplasm  which  produced  it. 
The  layer  of  protoplasm  not  only  varies  in  thickness,  being 
thickest  in  young  and  thinnest  in  old  cells,  but  is  never 
homogeneous.  Apart  from  the  nucleus,  chromatophores, 
and  granules  of  food  and  other  substances  contained  in  it, 
the  surface  of  the  protoplasmic  layer  in  contact  with  the 
cell-wall  is  differentiated  into  an  exceedingly  thin  living 
membrane,  the  ectoplast.  Within  the  living  protoplasm  are 
the  vacuoles — one  or  more  bodies  of  water  holding  in  solu- 
tion a  great  variety  of  compounds,  organic  and  inorganic 
— and  the  nucleus,  also  bounded,  like  the  living  protoplasm 
against  the  cell-wall,  by  cytoplasmic  membranes.  The  mem- 
branes bounding  the  vacuoles  are  called  tonoplasts.  The 
solution  filling  the  vacuoles  and  permeating  the  cell,  the  cell- 
sap,  is  the  active  agent  in  absorption,  although  its  composi- 
tion, and  therefore  its  action,  are  controlled  by  the  living  pro- 
toplasm, either  by  the  substances  formed  by  the  protoplasm 
and  transferred  to  the  cell-sap,  or  by  the  substances  permit- 
ted by  the  cytoplasmic  membranes  to  enter  or  pass  out  of  the 
cell,  the  vacuoles,  or  the  nucleus.  That  the  cytoplasmic 
membranes  do  exert  a  controlling  power  over  substances  in 


ABSORPTION  AND  MOVEMENT  OF  WATER  107 

solution  in  the  vacuoles  and  outside  the  cell  is  evident  from 
the  following.  Under  normal  conditions  the  protoplasm  is. 
slightly  alkaline,  the  cell-sap  slightly  acid!  If  the  cytoplas- 
mic  membrane  bounding  a  vacuole  (the  Vacuolenhaut, 
as  Pfeffer  calls  it)  were  permeable  to  all  substances,  and 
equally  permeable  in  both  directions,  this  difference  in 
chemical  reaction  could  exist  only  momentarily ;  it  could 
not  be  the  normal  condition.  Furthermore,  so  long  as  the 
cells  are  alive,  no  color  will  pass  from  clean  slices  of  beet  or 
of  red  or  black  cherry,  or  from  other  tissues  composed  of 
cells  containing  colored  sap  in  the  vacuoles ;  but  if  the  cells 
are  killed  by  immersion  in  hot  water  or  by  steam,  the 
color  will  rapidly  pass  out  into  the  water.  Again,  although 
harmless  coloring  solutions  will  pass  into  and  stain  the 
walls,  the  majority  of  such  solutions,  no  matter  what  their 
concentration,  will  not  pass  into  the  protoplasm  or  stain 
any  part  of  it,  so  long  as  the  cells  are  alive.  Some  few 
harmless  stains,  if  applied  in  sufficiently  dilute  solutions, 
may  be  employed  to  stain  living  protoplasm*  and  nuclei, t 
or  may  be  accumulated  in  the  vacuoles  t  of  living  cells. 

From  these  experiments  it  is  obvious  that  the  living  proto- 
plasm, especially  the  structurally  and  physiologically  differ- 
entiated layers  adjoining  the  cell- wall  and  bounding  the  vacu- 
oles and  nucleus,  do  exercise  some  control  over  the  dissolved 
substances  adjacent.  That  this  power  is  limited  is  shown  by 
the  above  experiments  with  staining  agents,  by  daily  experi- 
ence in  the  laboratory  with  the  various  poisons  employed 
as  fixing  agents  for  histological  purposes,  and  by  the 
sensitiveness  of  plants  to  the  soluble  substances  by  which 
they  are  surrounded.  If  the  cytoplasmic  membranes  could 
exclude  poisons,  at  the  same  time  allowing  nutritious  solu- 
tions to  enter  freely,  the  advantage  would  be  great.  § 

*  Pfeffer,  W.  Uber  Aufnahme  von  Anilinfarben  in  lebende  Zellen.  Unter- 
suchungen  aus  d.  bot.  Institut  zu  Tubingen,  Bd.  II. 

f  Campbell,  D.  H.    The  staining  of  living  nuclei.    Ibid. 

±  Pfeffer,  W.    L.  c. 

$  The  living  cell  may,  however,  control  the  osmotic  exchanges  taking 
place  between  itself  and  the  solutions  outside,  not  only  by  regulating  the 
composition  of  the  cell-sap,  but  also  by  changing  the  permeability  of  the 
cell-wall.  See  Nathansohn,  Zur  Lehre  vom  Stoffaustausch.  Ber.  d.  D. 
Bot.  Gesellsch.,  XIX.,  pp.  509-13,  1901. 


108  PLANT  PHYSIOLOGY 

The  living  plant-cell  is  then  a  series  of  concentric  perme- 
able ( or  partially  permeable )  membranes  of  different  compo- 
sition, properties,  and  needs,  surrounding  and  enclosing 
one  or  more  bodies  of  water  holding  various  substances 
in  solution.  Under  ordinary  conditions  the  density  of  this 
aqueous  solution  is  greater  than  that  of  the  solutions  out- 
side the  cell,  and  its  composition  is  different.  The  difference 
in  density  is  maintained  in  land  plants,  as  we  shall  see  from 
another  section  of  this  chapter,  by  the  loss  of  water  from 
the  leaves ;  the  difference  in  composition  is  due  to  the  activi- 
ties of  the  protoplasm.  In  constantly  submerged  aquatics 
the  means  of  maintaining  the  density  of  the  cell-sap  is  less 
obvious,  for  from  these  plants  no  water  is  lost  by  evapora- 
tion, and  no  concentration  by  this  means  takes  place.  Al- 
though the  water  in  the  cells  of  algae  may  not  be  lost  or 
changed,  the  same  result  as  regards  the  density  of  the  cell- 
sap  may  be  attained  in  another  way.  If  one  or  more  solu- 
ble substances  are  formed  by  the  protoplasm  and  trans- 
ferred to  the  cell-sap  faster  than  they  can  pass  out  into  the 
surrounding  water,  greater  density  will  be  maintained.  It 
is  upon  the  density  and  composition  of  the  cell-sap,  whether 
this  is  accumulated  in  larger  volumes  in  vacuoles  or  is 
uniformly  distributed  throughout  the  living  protoplasm, 
that  absorption  depends. 

DIFFUSION  AND  OSMOSIS 

Let  us  turn  aside  for  a  moment  from  the  living  cell  to 
consider  some  of  the  purely  physical  phenomena  and  princi- 
ples underlying  the  physiological  process  we  are  studying. 
If  two  equal  volumes  of  liquid  of  exactly  the  same  compo- 
sition— say,  two  volumes  of  pure  water — are  brought  into 
contact  with  each  other,  there  will  be  molecular  movements 
in  and  between  them,  but  there  can  be  no  change  in  the 
composition  or  pressure  or  any  other  quality  of  either. 
Suppose  five  grammes  of  common  salt  to  have  been  perfectly 
and  uniformly  dissolved  in  one  of  these  volumes  of  water 
before  the  two  were  brought  into  contact.  As  a  result  of 
bringing  the  two  volumes  of  water  into  contact  the  molecu- 
lar movements  in  and  between  the  two  volumes  will  produce 


ABSORPTION  AND  MOVEMENT  OF  WATER  109 

a  change  in  the  composition  of  both.  The  molecules  of 
salt,  being  free  to  move  not  only  through  the  one  volume 
in  which  they  were  dissolved,  but  also  into  and  through  the 
second  volume,  will  do  so,  and  there  will  therefore  come  to 
be  finally  an  equal  distribution  of  salt  molecules  in  the  two 
volumes,  each  volume  then  containing  two  and  one  half 
grammes  of  salt.  To  such  molecular  movement,  unaided  by 
stirring,  jarring,  or  other  mechanical  means,  the  name  diffu- 
sion is  given.*  Let  us  suppose  now  that  one  volume  of 
water  contained  five  grammes  of  common  salt  and  the 
other  five  grammes  of  any  other  salt,  say  potassium  nitrate. 
The  diffusion  of  the  common  salt  from  the  first  into  the 
second  volume  of  water,  and  of  the  potassium  nitrate  from 
the  second  into  the  first  volume  of  water,  would  be  at  practi- 
cally the  same  rate  as  into  pure  water.  Theoretically  there 
should  be  a  difference  in  rate ;  actually  there  is  no  difference 
which  can  be  detected.  There  would  presently  be  two  and 
one-half  grammes  of  common  salt  and  two  and  one-half 
grammes  of  potassium  nitrate  in  each  volume  of  water. 
But  if  we  had  five  grammes  of  common  salt  in  one  volume 
and  two  grammes  of  the  same  salt  in  the  other,  the  diffu- 
sion would  not  be  so  rapid,  although  the  mixture  would 
finally  be  as  perfect.  The  rate  of  diffusion  will  vary  with 
the  difference  in  the  proportions  of  salt  in  the  two  volumes, 
the  greater  the  difference  at  the  beginning  the  more  rapid 
the  diffusion;  the  nearer  the  proportions  come  to  being 
equal  the  slower  the  diffusion,  till,  ultimately,  with  equal 
proportions,  the  diffusion  ceases.  So  long  as  there  is  no 
chemical  action  of  one  salt  upon  another,  this  rule  applies 
as  well  to  solutions  containing  mixtures  of  salts  as  to 
solutions  of  single  salts. 

If  now  we  bring  together  two  equal  volumes  of  pure  water, 
only  interposing  a  permeable  membrane  ( bladder,  vegetable 
parchment,  cell-wTall)  between  them,  there  will  be  the  same 
molecular  movements  as  in  the  first  case  above  supposed, 
but  there  can  be  no  change  in  composition.  Similarly  there 

*  Illustrative  experiments  on  diffusion  are  described  in  Darwin  and 
Acton's  and  in  the  other  laboratory  manuals  of  plant  physiology  already 
referred  to  (p.  27). 


110  PLANT  PHYSIOLOGY 

will  be  movements  of  the  salt  as  well  as  of  the  water  mole- 
cules through  the  membrane  if,  in  the  other  cases,  we  sepa- 
rate the  two  volumes  of  liquid  by  a  permeable  membrane. 
As  we  have  already  seen  (p.  106),  this  form  of  movement, 
of  diffusion,  is  called  osmosis.  The  rate  of  osmotic  transfer 
will  vary  for  every  salt  according  to  the  difference  in  the 
proportions  of  the  salt  in  the  two  adjacent  liquids.  This 
difference  is  known  as  the  osmotic  pressure.  Furthermore, 
the  rate  of  movement  will  differ  with  the  salt,  with  the 
composition,  thickness,  etc.,  of  the  membrane,  and  with 
other  factors  ( e.  g:  the  relations  of  the  salts  to  one  another, 
with  the  dissociation,  etc. )  which  find  their  natural  place 
for  discussion  in  a  text-book  on  physics.* 

Turning  back  now  to  our  vegetable  cell — an  alga,  a  root- 
hair,  a  parenchyma  cell,  etc. — a  body  consisting  of  aqueous 
solutions  enclosed  in  and  permeating  concentric  membranes 
of  different  physical  and  chemical  properties,  we  see  that  we 
have  precisely  the  conditions  imagined  in  our  discussion  of 
the  purely  physical  phenomena.  The  cell-sap  is  a  solution 
greater  in  density  than  the  water  outside  the  cell  and  dif- 
fering from  it  in  composition.  In  consequence,  molecular 
movements  will  take  place  into  and  from  the  cell.  These 
movements  will  tend  to  reduce  the  density  and  modify  the 
composition  of  the  cell-sap.  Because  of  the  greater  density 
of  the  cell-sap — in  other  words,  because  of  the  smaller  pro- 
portion of  water  in  the  cell-sap  to  substances  dissolved  in 
it — water  molecules  will  pass  into  the  cell  through  the  cellu- 
lose wall,  the  cytoplasmic  membranes,  and  the  protoplasm, 
diffusing-  throughout  the  cell  as  well  as  in  the  vacuoles. 
Thus  the  density  of  the  cell-sap  will  be  lowered,  the  propor- 
tion of  water  to  substances  dissolved  in  it  will  be  raised, 
and  the  volume  of  the  cell-sap  will  be  increased.  But  if  the 
cellulose-wall  resist  any  increase  in  the  volume  of  the  cell- 
sap  and  of  the  cell,  the  cell-sap  will  be  subjected  to  pressure, 
the  protoplasm  will  be  forced  by  this  means  against  the 
cell-wall,  the  cell-wall  itself  will  be  stretched,  the  whole  cell 
will  be  in  a  state  of  tension,  will  be  plump,  will  be  turges- 

*  See,  for  example,  Ostwald's  Solutions,  translated  by  M.  M.  P.  Muir, 
London,  1891. 


ABSORPTION  AND  MOVEMENT  OF  WATER  111 

The  pressure  of  the  cell-sap  and  of  the  cell  brought 
about  by  this  means  is  called  turgor,  and  it  is  evidently  a 
very  important  factor  in  maintaining  the  form  of  the  cell 
and,  therefore,  comprehensively,  of  the  organs  and  of  the 
individual.  The  turgor  or  turgescence  of  the  cell  tends  to 
be  maintained  by  the  continued  absorption  of  water;  but 
unless  the  absorption  continually  exceed  the  loss  of  water, 
by  evaporation  or  otherwise,  there  will  be  no  turgor,  the 
plant  will  be  flabby,  wilted.  Absorption  of  water  can  be 
continued  only  by  keeping  the  density  of  the  cell-sap  always 
greater  than  that  of  the  water  outside. 

The  production,  control,  and  maintenance  of  this  physical 
condition  is  accomplished  in  part  by  the  living  protoplasm, 
in  part,  in  multicellular  plants,  by  the  osmotic  absorption 
of  water  from  one  cell  by  another.  In  the  latter  case,  water 
is  drawn  off  by  physical  means  only,  and  in  obedience  to 
unavoidable  physical  law,  by  the  cells  that  need  it.  In  the 
former  case,  the  living  protoplasm,  by  what  it  takes  from 
and  gives  to  the  cell-sap  in  respiration,  nutrition,  and  excre- 
tion, maintains  the  greater  density  of  the  cell-sap,  that  is, 
maintains  the  higher  proportion  of  dissolved  matter  to 
water  than  prevails  outside  the  cell. 

Upon  the  composition  of  the  cell-sap  depends  the  absorp- 
tion of  the  substances  dissolved  in  the  water  outside  the 
cell.  We  have  already  seen  that  a  dissolved  salt  will  pass 
through  a  permeable  membrane  into  another  volume  of 
water  which  contains  less  or  none  of  it,  and  that  the  rate  of 
osmosis  (or,  in  this  case,  of  absorption)  will  depend  upon 
the  salt,  the  difference  in  the  proportions  of  the  salt  in  the 
two  volumes  of  liquid,  upon  the  nature  of  the  membrane, 
etc.  The  cell — and  by  means  of  it,  the  plant — will  absorb  by 
osmosis  those  salts  which  occur  in  the  soil  and  in  water  in 
proportions  larger  than  in  the  plant.  For  example,  com- 
mon salt  will  be  absorbed  by  a  root-hair  or  by  an  algal  cell 
until  in  the  cell-sap  there  is  the  same  proportion  of  salt  to 
water  as  in  the  solution  outside  the  cell.  When  there  have 
come  to  be  the  same  number  of  molecules  of  salt  in  equal 
volumes  of  cell-sap  and  outside  water,  there  will  be  no  fur- 
ther absorption  of  salt,  and  although  the  movements  of 


112  PLANT  PHYSIOLOGY 

salt  molecules  will  continue,  there  will  be  no  accumulation 
of  these  either  within  or  without  the  cell.  Common  salt  will 
be  absorbed  by  the  cell,  therefore,  as  a  purely  physical 
necessity,  regardless  of  the  presence  of  the  other  salts  dis- 
solved in  the  cell-sap,  and  regardless  of  the  fact  that  it  is 
needed  and  used  by  the  cell  only  in  the  minutest  quantity 
if  at  all.  On  the  other  hand,  if  the  common  salt  were  used 
in  quantity  by  the  cell,  or  in  any  other  way  removed  from 
the  cell-sap  ( by  decomposition,  precipitation,  or  otherwise ) , 
the  proportion  of  common  salt  to  water  in  the  cell-sap 
would  always  be  lower  than  outside  the  cell,  there  would 
always  be  osmotic  pressure  inward,  the  molecules  of  salt 
would  constantly  force  their  way  into  the  cell  in  the  attempt 
to  attain  osmotic  equality  or  balance,  there  would  be  con- 
tinued absorption  of  common  salt,  and  the  rate  of  absorp- 
tion would  vary  with  the  osmotic  pressure.  Such  is  the  case 
with  salts  used  by  the  cell  or  otherwise  removed  from  their 
solution  in  the  cell-sap.  For  example,  the  nitrates  are,  as 
we  have  seen,  the  best  form  in  which  nitrogen  is  taken  in  by 
most  plants,  and  of  these  potassium  nitrate  is  perhaps  the 
most  common.  Potassium  nitrate  will  be  absorbed  by  the 
plant  with  an  avidity  proportioned  to  the  need  of  nitrogen, 
or — to  state  this  in  physical  instead  of  physiological  terms — 
at  a  rate  depending  upon  the  difference  in  the  proportions 
of  potassium  nitrate  within  and  without  the  cell.  If  the 
nitrate  be  decomposed  and  the  nitrogen  used  by  the  plant 
as  fast  as  it  is  absorbed,  the  osmotic  pressure  will  remain 
as  great,  the  absorption  will  continue  as  rapid,  as  at  the 
beginning.  Upon  the  amounts  needed,  used,  or  otherwise 
taken  out  of  the  cell-sap  by  the  living  protoplasm  will  de- 
pend the  amounts  of  different  salts  absorbed  by  plants  be- 
yond those  amounts  necessary  to  secure  uniformity  of 
composition  in  cell-sap  and  outside  water  if  no  salts  were 
consumed.  In  this  consists  the  so-called  "selective  power 
of  roots"  and  of  plant-parts  in  general. 

We  see,  then,  that  the  absorption  by  the  cell  of  those  sub- 
stances which  can  pass  through  cell-wall,  cytoplasmic  mem- 
branes, and  protoplasm  is  a  physical  necessity  whenever 
there  is  any  higher  proportion  of  these  substances  outside 


ABSORPTION  AND  MOVEMENT  OF  WATER  113 

the  cell  than  in  it.  When  there  is  no  difference  in  propor- 
tion, there  will  be  no  absorption  or  no  further  absorption. 
When,  by  the  vital  needs  and  activities  of  the  cell  or  of  the 
plant,  a  difference  is  maintained,  there  will  always  be  ab- 
sorption, proportioned  in  rate  to  the  difference,  propor- 
tioned in  amount  to  the  duration  of  the  difference.  This 
accounts  for  the  much  higher  percentage  of  potassium  than 
of  sodium  in  the  ash  of  marine  algae.  The  amount  of 
sodium  salts  absorbed  is  only  such  as  to  attain  the  osmotic 
balance  of  sodium  salts  in  the  cell-sap  and  in  sea  water, 
wherea  the  amount  of  potassium  salts  is  such  as  to  satisfy 
the  need  of  the  plant  for  potassium  and  for  the  elements 
associated  with  it  in  these  salts.  The  accumulation  of  io- 
dine in  marine  algae  is  due,  not  to  the  demand  of  these 
plants  for  iodine,  but  rather  for  the  element  or  elements 
with  which  the  iodine  is  combined  in  sea  water :  the  iodine 
is.  therefore,  removed  from  solution  in  cell-sap  and  accumu- 
lates in  insoluble  form  in  the  cell. 

THE  MEANS  OF  ABSORBING  NUTRIENT  SOLUTIONS 

From  the  foregoing  discussion  of  the  physical  principles 
underlying  the  absorption  of  nutrient  solutions,  we  can  now 
understand  how  an  alga  supplies  itself  with  adequate 
amounts  of  food-materials.  A  land  plant,  however,  in  addi- 
tion to  its  demand  for  other  food-materials,  must  regulate 
its  absorption  according  to  its  demand  for  water  to  make 
good  that  lost  by  evaporation.  WTe  must  now  consider  how 
the  land  plant  adapts  itself  to  the  conditions  prevailing  on 
land  and  successfully  employs  the  physical  means  of  absorb- 
ing the  nutrient  salts  in  the  soil. 

The  root  is  generally  regarded  as  especially  the  absorbing 
organ  of  higher  plants.  The  cells  on  the  surface  of  the  root, 
being  the  only  ones  which  are  directly  in  contact  with  the 
soil  particles  and  with  the  soil  water,  are  the  only  ones 
which  can  absorb  solutions  from  the  soil.  Only  the  young- 
est of  these  cells  have  walls  of  such  composition  and  thin- 
ness that  osmosis  can  take  place  rapidly.  Furthermore, 
owing  to  the  granular  nature  of  soils,  and  owing  to  the 
fact  that  the  water,  at  times  when  it  is  most  needed,  is  held 
8 


114  PLANT  PHYSIOLOGY 

longest  and  most  strongly  upon  the  soil  particles,  and  not 
between  them,  no  smooth  cylindrical  organ  of  the  size  of 
even  the  smallest  roots  will  be  able  to  bring  enough  of 
those  cells  capable  of  absorption  into  sufficiently  intimate 
contact  with  a  large  enough  number  of  soil  particles  to 
ensure  the  osmotic  absorption  of  water  from  the  soil  parti- 
cles into  the  root.  For  osmotic  transfer,  as  we  have  seen 
before  (p.  109),  both  of  the  two  liquids  concerned  must  be 
in  contact  with  the  permeable  membrane.  Furthermore,  as 
is  the  case  on  soil  particles,  water  strongly  held  as  a  thin 
film  over  an  irregular  surface  will  not  rapidly  move  from 
part  to  part  of  that  surface.  To  ensure  the  absorption  of 
much  water  from  such  a  surface,  there  must  be  the  most 
extended  proximity  possible  of  the  osmotically  active 
liquids.  The  permeable  membrane  must  therefore  cover  the 
irregular  surface  as  widely  and  as  closely  as  possible.  The 
most  intimate  contact  of  absorbing  cells  and  liquid  to  be 
absorbed  will  be  effected  when  hairs  of  such  size  and  length 
that  they  will  fit  the  soil  particles  develop  on  the  root. 

The  length,  thickness,  and  number  of  root-hairs  will  vary 
according  to  the  medium  in  which  they  develop,  and  also 
according  to  the  amount  of  water  given  off  by  the  plant. 
The  root-hairs  will  be  numerous  directly  in  proportion  to 
the  difficulty  of  getting  enough  water.  This  can  be  easily 
demonstrated  by  cultivating  young  seedlings  of  corn  with 
their  roots  in  moist  air,  in  soil,  and  in  water.  The  root- 
hairs  will  be  most  numerous  in  the  air,  less  in  the  soil,  and 
there  will  be  exceedingly  few  if  any  in  the  water.  The  length 
of  the  root-hairs  will  also  differ  strikingly ;  they  will  be 
longest  in  the  air,  shortest  in  the  water.  The  diameter  of 
the  hairs  is  necessarily  limited  by  the  size  of  the  epidermal 
cells  of  which  they  are  branches,  but  within  this  limit  the 
hairs  certainly  vary  according  to  the  size  of  the  soil  parti- 
cles among  and  around  which  they  must  grow.  That  the 
root-hairs  not  only  grow  between  the  soil  particles,  but 
actually  apply  themselves  very  closely  to  them,  is  abun- 
dantly proved  by  the  common  experience  of  up-rooting 
plants  grown  in  loose  soils.  When  such  plants  are  pulled 
up  gently,  numberless  soil  particles  of  minute  size  cling  to 


ABSORPTION  AND  MOVEMENT  OF  WATER  115 


the  roots,  held  there  by  the  root-hairs,  just  as  larger  lumps 
of  soil  are  held  by  the  root-branches. 

The  function  of  root-hairs  is  to  absorb  nutrient  solutions 
from  the  soil.  They,  and  their  physiological  equivalents, 
the  rhizoids  of  lower  plants,  are  the  chief  absorbing  organs 
of  larger  plants.  The  roots  themselves  are  for  the  conduc- 
tion of  the  solutions  absorbed  by  the  root-hairs,  and  also 
for  the  mechanical  support  of  the  whole  plant.  The  root- 
hairs  should  not  be  regarded  merely  as  structures  increasing 
the  surface  through  which  aqueous  solutions  are  absorbed 
by  roots ;  the  root-hairs  are  the  main  surface  through  which 
the  absorption  takes  place.  But  more  than  this,  they  are 
the  very  perfect  means  by  which  the  only  parts  capable  of 
copious  absorption — living  cells  bounded  by  thin  cellulose 
walls  and  containing  cell-sap  of  proper  composition  and 
density — are  brought  into  the  necessary  intimate  contact 
with  the  nutrient  solutions  adhering  to  the  irregular  sur- 
faces of  the  small  soil  particles.  In  other  words,  the  root- 
hairs  are  the  means  of  bringing  together,  so  that  they  are 
separated  only  by  a  thin  permeable  membrane,  two  aqueous 
solutions  of  such  osmotic  pressures  that  the  one  enclosed 
will  absorb  the  one  held  by  surface  attraction  (p.  105). 

It  has  been  estimated*  that  the  surface  of  a  root  is  in- 
creased 5  to  12  times  by  the  production  of  hairs.  From  what 
has  just  been  said,  and  because  the  hairs  are  bounded  by 
wralls  at  least  thinner  if  not  otherwise  more  permeable  than 
the  cells  between,  we  see  that  this  does  not  necessarily 
fairly  indicate  the  increase  in  absorbing  power  even  of  the 
part  producing  the  hairs.  This  may  be  more  than  5  to  12 
times  increased,  according  to  circumstances. 

The  life  of  a  root-hair  is  necessarily  brief.  Its  delicacy, 
and  the  fact  that  it  may  be  torn  and  broken  by  the  con- 
tinued forward  growth  of  the  part  where  it  is  borne,  favor 
this.  The  root-hairs  are  formed  by  the  outward  branching 
of  epidermal  cells  on  that  part  of  the  root  just  behind  the 
tip  which  has  almost  or  quite  ceased  to  grow  in  length.  If 
any  considerable  growth  in  length  does  occur  after  the  for- 

*  Schwarz,  F.  Die  Wurzelhaare  der  Pflanzen.  Untersuch.  aus  dem  botan. 
Institut  zu  Tubingen,  Bd.  I.,  p.  140. 


116  PLANT  PHYSIOLOGY 

mation  of  the  hairs,  these  must  surely  be  dragged  forward 
and  broken.  Secondary  growth  in  thickness  of  the  part  will 
crush  them.  Each  young  and  growing  root  or  root  branch 
is  covered  for  a  time  by  a  zone  of  root-hairs.  This  zone 
will  vary  in  breadth  according  to  growth  conditions.  The 
hairs  will  vary  in  length,  diameter,  and  number  according 
to  soil  conditions.  As  the  root  grows,  the  work  of  the 
older  and  less  effective  root-hairs  is  taken  up  by  the  younger 
ones  newly  formed  farther  forward  and  nearer  the  growing 
point.  In  this  way  new  soil-particles  are  relieved  of  their 
small  stores  of  water,  and  the  absorbing  surface  is  cor- 
related with  the  growth  of  the  plant  as  well  as  with  its 
demands  in  the  stationary  condition. 

THE  MEANS  OF  TRANSFER  OF  NUTRIENT  SOLUTIONS 

Similar  to  the  differences  between  cell-sap  and  soil- water 
in  density  and  in  composition,  which  enable  the  root-hairs 
to  absorb  water  and  dissolved  food-materials,  are  the  differ- 
ences in  the  density  and  composition  of  the  cell-sap  of  ad- 
jacent cells.  The  cell  with  denser  cell-sap  will  absorb  water 
from  its  neighbor  with  more  dilute  cell-sap,  the  cell-sap  with 
less  of  a  needed  food-material  or  food  will  absorb  from  one 
with  more.  Such  osmotic  transfers  are  inevitable  wherever 
miscible  solutions  or  liquids  of  different  densities  and  compo- 
sitions are  on  opposite  sides  of  a  permeable  membrane  and 
in  contact  with  it.  By  osmosis  the  distribution  of  food- 
materials  will  take  place  through  the  body  of  a  small  land 
plant,  P.  g.  a  fungus  or  a  liverwort,  and  in  submersed 
aquatics.  For  all  plants  not  subjected  to  the  loss  of  water 
by  evaporation,  and  in  the  bodies  of  most  land  plants  so 
small  and  so  simple  as  the  liverworts  and  the  fungi,  the 
rate  of  transfer  by  osmosis  alone  is  rapid  enough  to  ensure 
the  adequate  distribution  of  water  and  of  dissolved  foods 
and  food-materials.  Larger  land  plants  are  subjected  to 
such  losses  of  water  from  their  aerial  parts  that  osmotic 
transfer  is  too  slow  always  to  keep  pace  with  evapora- 
tion. In  plants  provided  with  special  organs  for  excret- 
ing water  (see  pp.  126-8)  these  organs  must  be  copiously 
supplied. 


ABSORPTION  AND  MOVEMENT  OF  WATER  117 

Evaporation  and  excretion,  taking  place  on  the  exposed 
surfaces  and  also  from  those  cells  bordering  on  air-passages, 
increase  the  density  of  the  cell-sap  of  the  cells  directly  con- 
cerned. These  draw  osmotically  upon  their  neighbors  for 
water  to  make  good  their  loss.  The  neighboring  cells  in 
their  turn  draw  upon  cells  still  more  remote  from  the  losing 
surface.  In  this  way  the  demand  for  water  is  developed  in 
cell  after  cell  away  from  the  surface.  To  meet  the  demand, 
water  is  transferred  from  cell  to  cell  toward  the  surface. 
Ordinarily,  cells  are  small  and  short,  and  though  their 
bounding  cellulose  membranes  and  their  component  proto- 
plasm may  be  freely  permeable,  water  can  move  more 
rapidly  in  response  to  other  than  osmotic  forces  if  only  the 
way  is  clear.  Through  living  cells  water  can  make  its  way 
best  by  osmosis,  but  as  water  will  pass  more  rapidly  through 
a  tube  in  which  no  filtering  membrane  is  interposed,  so  water 
in  the  plant  will  pass  more  rapidly  through  elongated  cells 
than  through  a  series  of  short  ones,  through  dead  and 
empty  tracheids  than  through  living  cells  of  the  same  di- 
mensions (other  things  being  equal),  and  through  con- 
tinuous ducts  than  through  a  succession  of  tracheids.  These 
stages  in  the  development  of  conducting  tissues  one  finds  in 
the  larger  erect  mosses,  in  the  Coniferae,  and  in  the  Angio- 
sperms.  The  most  perfect  development  of  a  vascular  sys- 
tem is  found  perhaps  in  twining  plants,  especially  those  of 
tropical  countries,  *  in  the  slender  stems  of  which  the  ducts 
are  large,  long,  and  no  thicker  walled  than  is  consistent 
with  the  necessary  mechanical  strength. 

Upon  the  vascular  tissues,  throughout  their  whole  length, 
parenchyma  cells  abut  directly.  In  the  root  these  paren- 
chyma cells  receive  more  and  more  water  so  long  as  fhe 
root-hairs  continue  to  absorb  any  from  the  soil,  and  pres- 
ently, not  being  able  to  expand  beyond  a  certain  point  by 
reason  of  the  pressure  of  their  neighbors,  they  are  obliged  to 
get  rid  of  the  excess  in  some  way  or  other.  Into  the  vascular 
tissues  of  the  root,  therefore,  the  parenchyma  cells  discharge 
the  water  and  dissolved  matters,  the  discharge  taking  place 
*  Schenck,  H.  Beitrage  zur  Biologie  der  Lianen.  Bot.  Mittheilungen 
aus  den  Tropen,  Bd.  II.,  1893. 


118  PLANT  PHYSIOLOGY 

in  the  direction  of  least  resistance,  that  is,  from  living  and 
turgid  cells  into  empty  tube-like  tracheids  and  ducts.  At 
any  point  in  the  plant  the  adjacent  parenchyma  cells  may 
absorb  water  from  the  vascular  tissues  just  as  the  root- 
hairs  absorb,  water  from  the  soil,  and  by  the  same  physical 
means.  Whether  there  are  continuous  columns  of  water  in 
the  ducts  or  not,  there  is  a  continuous  body  of  water  in  the 
walls  of  the  ducts,  and  so  the  withdrawal  of  water  at  any 
point  will  induce  a  movement  of  water  toward  that  point 
from  parts  better  supplied.  It  is  ordinarily  from  below  that 
water  is  drawn,  for  ordinarily  the  root-hairs  supply  the 
needs  of  the  whole  plant,  but  this  is  not  necessarily  the  case, 
for  through  the  conducting  tissues  water  will  pass  up  or 
down  according  to  circumstances. 

The  vascular  tissues  form  a  continuous  system,  often 
much  complicated  in  arrangement  but  proportionally  in- 
creased in  usefulness,  of  water-conveying  cells  and  vessels 
extending  from  base  to  tip  of  the  plant.  At  frequent  inter- 
vals the  bundles,  which  run  more  or  less  distinct  from  one 
another,  anastomose  and  thus  combine  the  vascular  tissues 
into  one  effective  system.  Branches  are  given  off  from  the 
main  channels,  so  that  buds,  leaves,  branches,  even  hairs 
(e.g.  glandular  hairs  otDrosera),  are  reached  by  the  con- 
ducting system.  By  this  means  all  the  parenchyma  cells, 
which  are  the  actively  living  cells  of  the  plant-body,  are 
supplied  directly  or  indirectly.  The  amount  of  water  and  of 
dissolved  matters  supplied  to  any  part  will  depend  upon  the 
demand,  upon  the  amount  lost  and  consumed.  For  ex- 
ample, two  adjacent  vascular  bundles,  running  to  two 
leaves,  will  convey  different  volumes  of  solutions  if  from  the 
one  leaf  more  water  evaporates  than  from  the  other,  or  if 
in  one  leaf  more  water  and  dissolved  matters  are  used  than 
in  the  other.  We  thus  see  that  the  amounts  transferred 
through  parts,  and  consequently  through  the  whole,  of  the 
vascular  system  are  dependent  upon  the  activities  of  living 
cells :  first,  upon  those  living  cells  which  absorb  nutrient 
solutions  from  the  soil  and  from  which  other  living  cells 
osmotically  absorb  them,  the  cells  abutting  upon  ducts 
and  tracheids  discharging  the  excess  of  water  into  these 


ABSORPTION  AND  MOVEMENT  OF  WATJER  119 


empty  spaces;  second,  upon  the  living  cells  of  the  aerial 
parts,  in  the  leaves,  branches,  and  stems,  in  which  the  solu- 
tions are  worked  over  and  from  which  the  water  is  given  oft'. 
The  living  parenchyma  cells  near  the  absorbing  cells  in  the 
roots,  and  the  living  parenchyma  cells  composing  the  food- 
making  tissues  in  other  parts  may  be  many  metres  apart. 
The  absorbing  and  consuming  tissues  of  herbaceous  plants 
are  usually  close  together ;  in  tall  trees  they  are  separated,  * 
but  are  farthest  from  each  other  in  some  of  the  "lianes." 
How  is  the  water  raised  from  the  IOWT  levels  at  which  it  enters 
the  vascular  bundles,  in  the  region  where  it  is  absorbed  from 
the  soil,  to  the  cells  needing  it  but  far  removed?!  This  ques- 
tion has  occupied  botanists  from  the  time  when  physiolog- 
ical experiments  were  first  undertaken  until  now.  Despite 
the  most  acute  study,  the  question— one  of  the  most  allur- 
ing and  important  in  botany — is  still  unanswered.  Hy- 
potheses, deserving  respectful  consideration  both  because  of 
their  reasonableness,  and  also  because  of  the  fame  of  their 
authors,  have  succeeded  one  another  in  the  text-books,  have 
been  accepted  and  then  discarded,  according  to  the  prevail- 
ing fashion.  As  Sachs  was  for  many  years  the  leading 
plant-physiologist,  so  his  idea,  laid  down  in  his  writings 
with  all  his  brilliant  power,  that  the  water  ascended  only 
through  the  walls  of  the  wood-elements,!  was  the  only  one 
echoed  by  the  smaller  text-books.  Then  followed  §  Godlew- 

*  From  Kerner  and  Oliver's  Natural  History  of  Plants  these  "certified 
estimates"  of  heights  and  lengths  are  quoted  :— 

Eucalyptus  amygdalina — 140-152  metres — page  722,  vol.  I.,  part  2. 

Sequoia  gigantea  79-142       "  "         "        "    "        "    " 

Calamus  Rotang  200  "  "      677,     "    "       "    " 

In  his  Silva  of  North  America,  Vol.  X.,  p.  141,  Sargent  makes  the 
following  statement  regarding  Sequoia  sempervirens :  "The  Redwood, 
which  is  the  tallest  American  tree,  probably  occasionally  attains  the  height 
of  four  hundred  feet  or  more.  The  tallest  specimen  I  have  measured  was 
three  hundred  and  forty  (340)  feet  high." 

f  While  this  book  is  in  the  press,  Copeland  is  publishing  in  the  Botanical 
Gazette,  vol.  34,  1902,  "The  rise  of  the  transpiration:  an  historical 
and  critical  discussion." 

t  Sachs,  J.  von.     The  Physiology  of  Plants,  Oxford,  1887,  pp.  241-242. 

§  Godlewski,  E.  von.  Zur  Theorie  der  Wasserbewegung  in  den  Pflanzen. 
Jahrb.  f.  wiss.  Bot.,  1884. 


120  PLANT  PHYSIOLOGY 

skTs  hypothesis,  on  the  face  of  it  much  more  reasonable, 
/but  not  directly  supported  by  experiment,(that  living  cells 
j adjoining  ducts  and  tracheids  exert  a  pumping  action^ 
By  dissolving  poisonous  substances  in  the  water  to  be  ab- 
sorbed by  the  plants  selected  for  examination,  Strasburger* 
attempted  to  demonstrate  that  living  cells  are  not  con- 
cerned. Sap-pressure  (see  pp.  127,  131)  was  for  a  time  con- 
sidered the  propelling  force,  but  the  absence  of  sap-pressure 
at  the  times  when  water  is  most  needed  is  sufficient  evidence 
against  this  notion.  The  varying  pressure  of  the  gases 
within  the  body  of  the  plant  was  supposed  to  be  the  answer 
to  the  question,  until  it  was  shown  that  gas-pressures  do 
not  vary  enough  and  rapidly  enough.  Then  came  the  era 
of  Jamin's  chains,  when  the  ducts  and  tracheids,  found  to 
contain  alternating  columns  of  air  and  water,  were  supposed 
to  furnish  the  tubes  through  which  these  pass.  Schwen- 
dener,f  Dixon  and  Joly,J  and  Askenasy§  have  contributed 
much  to  a  knowledge  of  the  physical  qualities  of  such 
chains,  but  no  one  has  succeeded  in  proving  that  they  have 
much  if  anything  to  do  with  water-transfer.  Eepeatedly 
botanists  have  returned,  from  lack  of  anything  better,  to 
the  idea  that  capillarity  conveys  the  water  through  the 
vascular  system ;  but  this  notion  is  inadequate  because  the 
vascular  system  of  many  plants  is  composed  of  tubes  so 
small  and  so  short  that,  although  the  capillary  force  is 
great,  the  resistances  are  also  so  great  that  a  sufficiently 
rapid  transfer  by  this  means  alone  is  inconceivable. 

From  all  the  contradictory  views  this  much  may  be  ex- 
tracted as  proved.  The  water,  which  certainly  permeates 
the  walls  of  the  elements  composing  the  vascular  system,  is 
also  contained  in  the  cavities  and  passes  through  the  cavi- 
ties in  the  direction  of  strongest  attraction,  conversely  of 
least  resistance.  That  the  water  does  actually  pass  into 

*  Strasburger,  E.  Bau  und  Verrichtung  der  Leitungsbahnen,  1891. 
tJber  das  Saftsteigen,  1893. 

f  Schwendener,  S.   In  Sitzungsberichte  der  Berliner  Akademie,  from  1886  on. 

J  Dixon  and  Joly.  On  the  ascent  of  sap.  Annals  of  Botany,  1894.  The 
Path  of  the  transpiration  current.  Ibid.,  1895. 

§  Askenasy.  IJber  das  Saftsteigen.  Verhandlung  d.  naturh.  Vereins  in 
Heidelberg,  1895. 


ABSORPTION  AND  MOVEMENT  OF  WATER  121 

and  through  the  cavities  of  ducts  and  tracheids  is  demon- 
strated by  using  a  solution  of  gelatine,  which  melts  at  a 
temperature  so  low  as  not  to  injure  the  plant,  but  which  is 
solid  at  ordinary  temperatures.  For  example,  gelatine 
melted  at  about  30°  C.  will  be  taken  into  the  freshly-cut 
butt  of  an  amputated  branch  and  can  then  be  hardened  by 
plunging  into  water  at  20°  C.  If  now  the  branch,  with  the 
butt  freed  from  superfluous  adherent  gelatine,  is  stood  up  in 
a  jar  of  cool  water  its  leaves  wrill  wither.  They  will  recover, 
however,  if  placed  in  water  warm  enough  to  melt  out  the 
gelatine.  This  experiment  is  significant  in  two  ways  at 
least.  First,  solid  gelatine  permeated  with  water  is  no  bar 
to  osmotic  transfer  of  aqueous  solutions,  while  paraffine,  or 
any  similar  material  which  is  impermeable  to  water  but 
might  also  be  used  to  fill  the  cavities  of  the  vessels,  would 
stop  osmosis.  We  see  then  that  osmotic  transfer,  even  if  it 
could  take  place  in  the  ducts  as  it  does  between  the  paren- 
chyma cells,  is  too  slow  for  the  conduction  of  more  than 
small  volumes  of  solutions  and  for  short  distances.  Second, 
if  the  solidified  gelatine  is  properly  removed  from  the  sur- 
face of  the  butt  of  the  branch  experimented  upon,  little  or 
no  penetration  of  gelatine  into  the  walls  of  the  ducts  having 
taken  place^  the  permeability  and  conducting-power  of  the 
walls  will  be  only  very  slightly  diminished  if  impaired  at  all. 

Further  than  this  no  positive  assertions  can  be  made. 
The  water  certainly  ascends,  mainly  in  the  cavities  of  ducts 
and  tracheids,  though  also  in  the  walls,  and  whenever  the 
physical  conditions  demand  it,  water  and  dissolved  salts  will 
be  drawn  from  the  phloem  as  well  as  from  the  xylem.  The 
main  path  for  the  transfer  of  the  solutions  of  food-materials 
is  the  wood ;  for  the  transfer  of  the  solutions  of  elaborated 
foods,  the  phloem  or  bast-portion  of  the  vascular  bundles. 
The  physical  force  needed  to  raise  the  water  is  still  unknown. 

Of  the  various  views  regarding  the  means  of  transfer  to 
which  reference  was  made  on  pages  119  and  120,  one  is  further 
from  disproof,  perhaps  also  from  proof,  than  the  others.  This 
is  the  one  put  forth  by  Godlewski,  *  and  advocated  in  more 

*  See  Pfeffer,  Pflanzenphysiologie,  Bd.  I.,  p.  208 ;  Engl.  transl.,  I.,  pp. 
220  et  seq. 


122  PLANT  PHYSIOLOGY 

or  less  modified  form  by  a  number  of  other  authors.  Accord- 
ing to  this  view,  the  living  cells  which  are  always  found  to 
be  the  close  neighbors  of  ducts  and  tracheids  participate 
actively  in  raising  water  from  roots  to  leaves.  Apart  from 
the  anatomical  relationship  of  these  living  and  lifeless  ele- 
ments, which  suggests  that  the  living  cells  may  aid  in  as 
well  as  influence  the  movement  in  the  lifeless  ducts  and 
tracheids,  it  is  definitely  proved  by  experiment  that  it  is  the 
youngest  wood,  that  is,  the  wood  containing  the  most  and 
the  most  active  living  cells,  which  transfers  most  water  and 
does  it  most  rapidly.  The  method  of  proof  consists  in  using 
solutions  of  harmless  coloring-matters  not  fixed  by  living 
cells  ( e.  g.  Indigo-carmin,  Anilin  Blue,  etc. ) .  If  amputated 
branches  are  placed  in  such  solutions  the  path  of  transfer 
will  be  indicated  by  the  staining  of  cell-walls,  and  if  the  ex- 
periment is  not  prolonged,  the  stain  will  be  found  highest 
in  the  youngest,  that  is,  the  best-conducting  wood.  The 
"sap-wood"  conducts  most  if  not  all  of  the  water.  On  the 
other  hand,  the  "  heart- wood  "  conducts  little  or  none.  The 
heart-wood  not  only  contains  fewer  living  cells — the  oldest 
heart- wood  none  at  all — but  its  permeability  diminishes  as 
the  infiltration  of  the  walls  with  coloring,  hardening,  pre- 
serving, and  other  substances  progresses.  In  the  youngest 
wood,  where  there  are  the  most  living  cells,  the  maximum 
transfer  of  water  takes  place. 

Against  the  idea  that  living  cells  are  actively  concerned  in 
raising  water  from  root  to  leaf  are  the  conclusions  drawn 
by  Strasburger  from  his  experiments.  The  following  will 
serve  as  an  Illustration  of  one  line  along  which  he  experi- 
mented. *  A  specimen  of  Acer  platanoides  twenty-one  metres 
high  was  obliquely  sawed  through  at  the  base,  a  strong 
stream  of  water  playing  constantly  into  the  cut  as  the  saw- 
ing progressed.  The  tree  was  then  placed  with  the  butt  in 
water.  After  the  lapse  of  a  half-hour  it  was  hoisted,  the 
cut  surface  smoothed  with  a  sharp  knife,  and  then  lowered 
into  a  5%  solution  of  copper  sulphate.  In  two  weeks  it  took 
up  nearly  30  litres  of  this  liquid  and  the  presence  of  the 
copper  could  be  demonstrated  up  to,  but  not  in  the  finest 
*  Strasburger,  E.  Bau  und  Verrichtung  der  Leitungsbahnen,  p.  617,  1891. 


ABSORPTION  AND  MOVEMENT  OF  WATER  123 


and  highest  branchlets.  The  rapid  ascent  of  so  large  a 
volume  of  poisonous  liquid  is  alleged  to  prove  that  living 
cells  are  not  necessary  to  the  transfer.  One  very  strong 
objection  to  this  conclusion  is  this  .-—although  any  cell  into 
which  even  a  small  amount  of  copper  penetrates  will  be 
poisoned  and  killed  thereby,  the  cell  next  above  will  not 
cease  its  activity  until  it  in  turn  absorbs  and  is  poisoned  by 
the  copper.  Furthermore,  it  is  weU  known  that  water, 
which  already  permeates  all  cell-walls,  will  ascend  faster 
than  substances  dissolved  in  it  but  not  permeating  cell- 
walls.  The  poison  will  ascend  less  rapidly  than  its  solvent 
because  the  copper-salt  will  be  taken  up  by  the  cell- wall,  and 
will  diffuse  osmotically  through  the  cell.  If  then,  living  cells 
do  take  an  active  part  in  the  transfer  of  water,  the  ones 
above  and  not  killed  by  the  copper  can  still  pull  up  the 
solution  though  to  then1  own  ultimate  undoing,  and  they 
will  pull  up  water  faster  than  copper  salt. 

Whether  living  cells  are  actively  concerned  in  water-trans- 
fer or  not,  the  popular  idea  of  the  lifetime  of  a  cell  must  be 
modified  somewhat.  Those  trees  which  form  no  "  heart- 
wood"  and  in  which  living  cells  may  be  found  quite  to  the 
centre  (e.  g.  Beech  and  Birch),  and  the  Palms  and  other 
Monocotyledons  which  do  not  increase  in  thickness,  offer 
striking  examples  of  the\age  attained  by  living  ceUs.  Stras- 
burger*  reports  finding  living  cells  in  large  numbers  almost 
to  the  centre  in  sections  of  seventy-year  old  beech  trees. 

In  the  wood  of  trees  growing  hi  regions  with  pronounced 
seasonal  differences  there  are  seasonal  as  well  as  age  differ- 
ences. The  "  annual  rings"  are  divisible  into  so-called 
'•spring- wood"  and  "autumn- wood,"  although  the  latter  is 
formed  long  before  autumn.  The  anatomical  differences 
which  distinguish  these  layers  of  the  annual  ring  from  one 
another  are  accompanied  by  differences  in  their  conducting 
power.  Spring-wood  is  formed  at  the  time  when  sap-pressure 
is  greatest,  when  the  opening  of  buds  is  followed  by  the  ex- 
pansion of  the  leaf  and  other  surfaces  from  which  wrater  can 
be  'given  off,  when  the  plant  resumes  all  at  once  the  activi- 
ties which  have  been  suspended  for  a  season,  and  when  most 

*  L.  c.,  p.  534. 


124  PLANT  PHYSIOLOGY 

food  as  well  as  most  water  must  be  carried  to  the  active 
parts.  Spring- wood  conducts  better  than  autumn  wood, 
although  according  to  Strasburger,  *  single  rows  of  cells 
formed  last  in  the  autumn-wood  possess  higher  conducting 
powers  than  those  formed  earlier.  This  he  regards  as  con- 
tributing to  a  better  connection  between  the  succeeding 
rings,  and  this  is  especially  necessary  because  the  new  bundles 
for  the  forming  and  growing  parts  must  be  adequately  sup- 
plied with  liquid  while  the  young  spring-wood,  with  which 
they  connect,  is  attaining  effective  dimensions.  (For  the 
cause  of  "annual  ring"  formation,  see  pages  191-4.) 

Perhaps  in  all  plants,  certainly  in  many  plants  growing  in 
desert  regions  and  in  places  where  there  is  a  distinct  dry 
season,  tissues  are  developed  in  which  water  may  be  stored 
and  kept  for  a  long  time,  in  spite  of  the  dryness  of  the  sur- 
rounding air.  It  is  quite  possible  that  the  wood-nbres, 
present  always  in  the  xylem  of  the  vascular  bundle  and 
sometimes  numerous  there,  serve  as  temporary  holders  of 
water  at  the  same  time  that  they  contribute  to  the  me- 
chanical strength  of  the  plant.  After  collenchyma  has 
served  its  first  purpose  in  strengthening  the  rapidly  growing  : 
parts  in  which  it  differentiates  so  early,  its  thickened  and  ' 
chemically  modified  walls,  as  well  as  the  living  cells  which 
formed  them,  retain  water  with  considerable  power,  t  The 
most  striking  examples  of  water-storing  tissues  are  to  be 
found  among  desert  plants  (£.#•.  in  the  Cactacea*  and 
Euphorbiace*),!  and  in  the  leaves  of  Sphagnacea\  which 
live  under  exactly  opposite  conditions.  The  possession  by 
swamp-plants,  especially  those  living  in  undrained  swamps 
and  in  bogs,  of  characters  found  otherwise  only  in  desert- 
plants  has  been  remarked  by  a  number  of  authors.  This 
may  be  due  to  the  plants  trying,  by  reducing  evaporation 
and  therefore  the  need  of  absorption,  to  avoid  absorbing  in 
excess  any  of  the  poisonous  matters  (humic  acid,  etc.)  in 

*  7.  c..  p.  592. 

f  Miiller,  C.  Beitrag  zur  Kenntniss  der  Formen  des  Collenchyms. 
Berichte  der  Deutsch.  Bot.  Gesellschaft.  1890. 

i  See  Goebel's  Pflanzenbiologische  Schilderungen  find  Volkens's  Flora  der 
Mgyptisch-arabischen  Wiiete,  1887. 


ABSORPTION  AND  MOVEMENT  OF  WATER  125 

iswamps,  or  it  may  be  due  to  the  actual  difficulty  of  ab- 
sorbing water.* 

SECRETION 

Before  leaving  the  subject  of  the  osmotic  phenomena  in  the 
plant-body  to  discuss  those  of  water  vaporization  and  gas 
exchange,  those  osmotic .  processes  which  result  in  the  re- 
moval of  material  from  the  plant  should  be  mentioned.  If 
there  is  absorption  by  means  of  osmotic  currents  set  up  and 
maintained  because  there  are  smaller  proportions  of  water 
and  of  various  other  substances  inside  than  outside  the  cell, 
there  must  be  excretion  by  the  same  means  whenever  the 
opposite  is  true.  Such  excretion  does  take  place ;  there  are 
exosmotic  as  well  as  endosmotic  currents.  They  are  very 
different,  however,  in  amount,  rate,  and  character.  By  the 
root-hairs  of  higher  plants  water  and  dissolved  substances 
are  endosmotically  absorbed.  Minute  quantities  of  a  number 
of  substances  are  exosmotically  excreted  also  by  the  root- 
hairs.  Since  Sachs's  classic  experiment  in  growing  roots 
in  contact  with  polished  marble  plates,  t  it  has  been  known 
that  roots  can  exert  a  corrosive  action  on  such  solid  mat- 
ters as  they  touch.  Recent  experiments  by  CzapekJ  have 
shown  that  the  principal  substances  diffusing  from  roots 
are  mainly  carbon-dioxide  (passing  out  as  carbonic  acid, 
H.,CO3),  phosphoric,  hydrochloric,  sulphuric,  and  phormic 
acids  and  their  salts,  preeminently  substances  which  would 
aid  the  plant  to  obtain  needed  food-materials  from  the  soil. 

Besides  the  excretion  from  roots,  the  secretions  in  the 
various  glands  and  reservoirs  are  dependent  upon  exosmotic 

*  Cowles,  H.  C.  The  ecological  relations  of  the  vegetation  of  the  sand 
dunes  of  Lake  Michigan.  Botanical  Gazette,  vol.  27,  1899.  Schimper,  A. 
F.  W.  Pflanzengeographie  auf  phisiologischer  Grundlage,  1898. 

t  Sachs,  J.  von.  Auflosung  des  Marmors  durch  Mais-Wurzeln.  Bota- 
nische  Zeitung,  1860.  Lectures  on  the  Physiology  of  Plants,  pp.  262, 
263,  Oxford,  1887. 

J  Czapek,  F.  Zur  Lehre  von  den  Wurzelausscheidungen.  Jahrb.  f.  wiss. 
Botanik,  Bd,  29,  1896.  See  also  Sistini  in  Atti  di  Soc.  Tosc.  di  nat., 
Proc.  Verb.,  1899  (reviewed  in  Just's  Jahresbericht,  Bd.  27,  2te  Abth., 
p.  194),  who  says  roots  convert  feldspar  into  clay,  working  at  least 
four  times  as  fast  as  the  weather. 


126  PLANT  PHYSIOLOGY 

movements.  Nectaries  are  special  organs  on  the  surface  of 
which  sugar  is  abundantly  produced.  What  causes  the  for- 
mation and  excretion  of  sugar  by  the  cells  of  nectaries  is 
not  known,  but  given  the  sugar  on  the  surface  of  a  nectary, 
the  excretion  of  water  to  dissolve  this  is  inevitable.  *  Why 
this  sugary  solution  is  not  resorbed  is  also  unknown.  Ap- 
parently no  clear  idea  of  the  action  of  nectaries  can  be  had  un- 
til the  physiological  chemistry  of  these  organs  is  worked  out. 

The  accumulation  of  resins  in  the  intercellular  resin-reser- 
voirs of  the  Conifers,  etc.,  and  the  incrustations  of  lime 
and  iron  on  the  surfaces  of  various  plants,  are  accounted 
for  in  the  following  way.  Certain  substances  elaborated  or 
formed  as  by-products  by  glandular  cells,  are  excreted  os- 
motically  into  intercellular  spaces,  or  upon  the  surface,  or 
under  the  cuticula  (in  certain  hairs),  there  undergoing  such 
chemical  change  that  they  are  no  longer  capable  of  osmotic 
movement  in  water.! 

The  excretion  of  fluid  water  by  many  plants  is  also  ac- 
complished by  purely  physical  means.  Water  vapor  is  uni- 
versally given  off  by  land  plants  (p.  136),  but  the  escape 
of  fluid  water  is  a  less  frequent  and  regular  occurrence.  In 
nectaries  the  passage  of  fluid  water  from  the  cell  is  due  to 
osmotic  pressure,  the  attraction  of  the  excreted  sugar.  In 
other  cases,  on  the  contrary,  fluid  water  is  excreted,  not 
because  of  the  attraction  (osmotic  suction)  of  substances 
outside  the  cell  but  because  of  the  pressure  (turgor,  p.  110) 
within  the  cell. 

Turgor  will  develop  in  a  cell  whenever  the  cell-sap,  be- 
cause it  contains  a  higher  percentage  of  dissolved  salts 
than  the  water  outside,  can  absorb  water.  The  volume 
of  the  cell-sap  and  of  the  enclosing  protoplasm  tends  to 

*  Wilson,  W.  The  cause  of  the  excretion  of  water  on  the  surface  of  necta- 
ries. Untersuchungen  aus  dem  Bot.  Institut  zu  Tiibingen,  Bd.  I.,  1881. 
Schimper,  A.  F.  W.  Wechselbezienhungen  zwischen  Pflanzen  und  Ameisen, 
1888. 

i  See  Pfeffer,  Pflanzenphysiologie,  Bd.  I.,  pp.  115,  116  501;  Engl.  transl., 
pp.  129,  500.  Kohl,  F.  G.  Kalsalze  und  Kieselsaure  in  der  Pflanze,  1889. 
Giesenhagen,  C.  Die  radialen  Strange  der  Cystolithen  von  Ficus  elastica. 
Berichte  der  Deutsch.  Bot.  Gesellsch.,  1891.  Tschirch,  A.  Die  Harze  und 
die  Harzbehalter.  Berlin,  1900. 


ABSORPTION  AND  MOVEMENT  OF  WATER  127 

increase  with  the  absorption  of  water.  Such  increase  in 
volume  is  only  feebly  resisted  by  the  mechanically  weak 
protoplasm.  It  can  be  resisted  only  by  the  cell-wall,  a 
strong,  elastic,  permeable  membrane,  composed  of  one  of  the 
celluloses.  Turgor  and  sap-pressure  result  from  the  resist- 
ance by  the  cell-wall  to  increase  in  volume.  If  the  sap- 
pressure  in  a  cell  becomes  greater  than  the  retaining  power 
of  the  wall,  something  will  change.  The  absorption  of  water 
may  be  stopped  by  modifying  the  composition  of  the  cell- 
sap,  by  exosmosis  of  the  osmotically  active  salts  to  other 
cells,  or  by  chemical  change  of  the  salts  in  the  sap ;  or  water 
may  pass  out  through  the  wall  in  a  direction  of  less  pres- 
sure, either  into  adjacent  cells  or  out  upon  the  surface  of  the 
organ.  If  none  of  these  things  occur  and  absorption  con- 
tinues, the  cell-wall  will  break. 

The  cortical  parenchyma  cells  in  the  root,  in  the  region 
where  absorption  through  the  hairs  is  taking  place,  are 
under  sap-pressure — whence  the  misleading  name  of  root- 
pressure — and  consequently  force  water  into  the  conducting 
elements,  tracheids  and  ducts,  with  which  they  are  in  con- 
tact. The  same  process  underlies  the  action  of  water-pores. 

Certain  weather  conditions  favor  the  excretion  of  water  by 
making  it  possible  to  develop  the  necessary  sap-pressure. 
When  the  air  is  warm  and  moist  above  a  warm  damp  soil, 
there  will  be  copious  absorption  through  the  roots  and  pro- 
portionally little  loss  of  water  from  the  upper  parts  of  the 
plant  by  evaporation.  Sap-pressure  necessarily  develops, 
and  if  these  conditions  continue,  the  pressure  will  presently 
exceed  the  retaining  power  of  some  or  many  cells,  water 
will  either  filter  through  or  break  through  the  cell-walls. 
If  it  break  through,  a  wound  is  formed,  and  the  escape  of 
liquid  from  it  is  bleeding  (see  pp.  130-36).  Such  wounds 
are  not  altogether  uncommon.* 

The  filtering  of  water  under  pressure  through  cell-walls 
does  not  take  place  indiscriminately,  for  the  permeability  of 
the  walls  of  the  cells  composing  the  different  tissues  is  not 

*  See  Pfeffer,  Pflanzenphysiologie,  Bd.  I.,  pp.  255  et  seq.;  Engl.  transl., 
I.,  pp.  272  et  seq.  Strasburger,  E.  Bau  und  Verrichtung  der  Leitungs- 
bahnen,  1891. 


128  PLANT  PHYSIOLOGY 

equal.  Thus,  as  a  rule,  the  walls  of  superficial  cells  are  so 
water-proofed,  either  by  chemical  change  or  by  infiltration 
of  the  cellulose,  that  water  will  be  pressed  out  of  other  cells 
before  it  will  pass  from  them.  The  so-called  water-pores— 
such  as  occur  on  the  garden  nasturtium — are  merely  the 
openings  on  the  edges  of  the  leaves  of  cavities  at  the  tips  of 
vascular  bundles.  The  thin-walled  cells  bordering  upon  the 
ducts  and  tracheids  of  these  bundles  squeeze  out  water  into 
them,  the  water  makes  its  way  toward  the  surface,  escapes 
into  the  cavity,  and  finally  passes  out  through  the  pore. 
The  excretion  of  water  can,  however,  be  observed  on  many 
plants  not  provided  with  such  highly  developed  filtering 
organs.  On  the  leaves  of  grasses  grown  under  glass  in  the 
laboratory,  and  on  the  filaments  or  erect  fruiting  bodies  of 
various  fungi  similarly  cultivated,  water  will  collect  in  drops 
whenever  the  substratum  is  so  moist  that  checking  the 
evaporation  will  raise  the  sap-pressure  to  the  filtering 
point.  This  occurs  regularly  in  grass-plats  at  night.  The 
"dew"  there  formed  is  mainly  expressed  water  rather  than 
moisture  condensed  from  the  air. 

A  considerable  number  of  plants,  especially  those  growing 
in  damp  tropical  regions,  rid  themselves  of  superfluous  water 
by  means  of  living  glandular  hairs  on  the  surface,  usually 
the  under  surfaces  of  leaves.  According  to  Haberland*  these 
glandular  hairs,  to  which  he  gives  the  name  hydathodes, 
press  out  liquid  only  when  living. 

The  liquid  passing  out  of  water-pores  and  excreted  by 
hydathodes  is  usually  a  very  dilute  solution,  mainly  of 
mineral  substances,  with  little  or  no  sugar  or  other  organic 
compounds.  These  organs  are  therefore  quite  different  as  to 
their  products  from  nectaries. 

Other  water-excreting  glands  are  not  uncommon.  The 
accumulation  of  water  in  the  pitchers  of  Nepenthes,  Sarra- 
cenia,  Darlingtonia,  the  secretions  on  the  hairs  of  Drosera, 
and  on  the  leaves  of  Dionea,  are  due  to  the  action  of  water- 
glands.  The  cells  composing  these  glands  change  not  only 

*  Haberland,  G.  In  Sitzungsberichte  d.  Akad.  d.  Wissensch.,  Math-phys. 
Klasse,  Bd.  103,  Abth.  1,  Wien,  1894;  ibid.  Bd.  104,  1895;  also  Jahrb. 
f.  wiss.  Botanik,  Bd.  30,  1897. 


ABSORPTION  AND  MOVEMENT  OF  WATER  129 

the  rate  but,  as  we  have  already  seen  (page  83),  the  kind 
of  activity  in  accordance  with  certain  stimuli.  On  the  hairs 
of  Droseraj  and  on  the  leaves  of  other  carnivorous  plants, 
the  sugary  secretion  which  is  used  for  attracting  and  cap- 
turing prey  may  be  more  like  that  formed  by  nectaries 
(page  126)— due,  so  far  as  the  water  is  concerned,  to  os- 
motic suction  rather  than  to  active  pressing  out.  The  fill- 
ing up  of  the  pitchers  of  the  pitcher-plants  is  much  more 
likely  to  result  from  active  excretion  of  water. 

The  mechanics  of  the  secretions  commonly  taking  place  on 
the  surface  of  stigmas  are  probably  identical  with  those  of 
nectaries,  although  it  must  be  seen  that  the  first  secretion  of 
sugar,  and  of  the  water  which  carries  it,  may  in  both  cases 
be  accomplished  by  active  pressing  out  of  these  substances 
by  the  living  cells  forming  the  surface  of  the  gland.  When 
sap-pressure  is  high  in  the  body  of  the  plant  or  in  the  root- 
hairs  themselves,  the  dissolved  substances  passing  out 
through  root-hairs  may  be  pressed  out  mechanically  by  the 
vital  activity  of  the  living  protoplasm,  as  well  as  by  the 
difference  in  the  osmotic  pressures  within  and  without 
the  cell  (page  125). 

The  substances  passing  out  exosmotically  or  by  other 
pressure  from  the  cells  are  seldom,  if  ever,  such  as  con- 
tribute directly  to  the  formation  of  protoplasm.  Proteids, 
albuminoids,  and  the  like,  remain  in  the  cells  in  spite  of 
the  differences  in  proportional  composition  of  the  liquids 
within  and  without  the  cells.  Though  some  of  these  sub- 
stances may  be  soluble,  none  is  freely  diffusible.  According 
to  the  differences  in  diffusibility,  soluble  substances  have 
been  divided  into  crystalloids  and  colloids,  respectively  sub- 
stances readily  and  tardily  diffusing.  It  is  now  known,  con- 
trary to  former  supposition,  that  colloids  may  crystallize. 
For  this  reason  the  names  are  inapt  and  misleading.  The 
hypothetical  explanation  of  the  feeble  diffusibility  of  colloi- 
dal substances  is  this :  the  molecules  of  these  highly  complex 
compounds  are  so  large  (e.  g.  egg-albumen,  which,  according 
to  Lieberkuhn,  may  have  the  formula  C^H^N^OJ^*)  that 

*  Loew  and  Bokorny.     Die   chemische    Kraftquelle  im   lebenden    Proto- 
plasma.    Munich,  1882. 
9 


130  PLANT  PHYSIOLOGY 

they  cannot  pass  through  the  spaces  between  the  molecules, 
or  groups  of  molecules,  of  other  substances.  On  the  other 
hand,  a  colloidal  membrane  is  no  bar  to  the  diffusion  of 
crystalloids,  for  though  its  molecules  and  groups  of  mole- 
cules are  large,  it  is  supposed  that  the  spaces  between  are 
large  enough  for  smaller  molecules  to  pass  through.  Thus, 
apart  from  Hertwig's  conception  ( p.  7 )  that  the  living  proto- 
plasm is  a  definite  structure  and  not  a  substance  or  mixture 
of  substances  merely,  we  have  a  reason  why  the  protoplasm, 
while  permitting  the  free  passage  of  wrater  and  of  the  sub- 
stances dissolved  in  it,  remains  enclosed  within  the  cell-wall, 
although  by  the  absorption  of  much  water,  or  for  other  rea- 
sons, the  density  of  the  protoplasm  may  be  greatly  reduced. 
In  this  sense  the  living  cell  is  an  apparatus  that  permits 
endosmosis  while  preventing  exosmosis.  In  addition  to  the 
failure  of  protoplasmic  (7.  e.  of  colloidal)  substances  to 
pass  through  the  cell- wall  because  of  the  size  of  their  mole- 
cules, the  living  protoplasm  (see  p.  107)  prevents  by  its 
bounding  membranes  the  exosmosis  of  dissolved  coloring 
and  of  some  other  substances  contained  in  the  vacuoles. 

SAP-PRESSURE  AND  BLEEDING 

The  transfer  of  single  cells  containing  osmotically  active 
substances  in  abundance — for  example,  ripe  pollen-grains — 
into  pure  \vater,  or  to  an  aqueous  solution  of  too  low  den- 
sity, will  cause  the  cells  to  swell  and  finally  to  burst,  in 
consequence  of  turgor-pressure  which  finally  ruptures  the 
cell- walls.  Such  cells  would  not  swell  and  burst  in  air,  or  in 
a  solution  nearly  or  quite  equalling  the  cell-sap  in  density. 
Similarly,  the  accumulation  of  osmotically  active  substances 
in  some  of  the  cells  of  a  multicellular  plant  which  can  ab- 
sorb water  in  abundance,  will  result  in  developing  pressure 
—turgor-pressure— in  those  cells.  These,  exerting  pressure 
upon  neighboring  cells,  will  transmit  the  mechanical  force, 
often  for  a  considerable  distance,  and  it  may  ultimately  be 
exerted  directly  upon  the  soil  or  other  material  surrounding 
the  plant.  The  materials  causing  the  development  of  pres- 
sure may  pass  by  osmosis  to  the  cells  against  which  pressure 
is  exerted.  Thus,  though  the  pressure  in  one  cell  or  several 


ABSORPTION  AND  MOVEMENT  OF  WATER  131 

may  thereby  be  reduced,  the  means  of  developing  turgor 
will  be  extended  and  the  total  pressure  of  the  organ  or  of 
the  plant  may  remain  the  same.  The  turgor-pressure  may 
also  be  reduced  by  the  living  cells  which  abut  on  the 
lifeless  ducts  and  tracheids,  pressing  out  water  into  these 
otherwise  empty  shells.  Continuing  this  process  of  excre- 
tion into  the  wood  elements  may  result  in  pressure  develop- 
ing in  them  also.  This  pressure  in  lifeless  cells  may  justly 
be  called  sap-pressure  in  distinction  from  turgor-pressure, 
which  is  possible  only  in  living  cells  or  in  an  apparatus 
similarly  constructed. 

Perennial  plants  in  temperate  climates  exhibit  all  of  these 
phenomena  each  year  with  the  return  of  spring.  When  there 
are  again  sufficient  warmth  and  water,  the  cells  in  which 
starch  and  other  reserve  foods  were  stored  for  the  winter  are 
awakened  to  new  life,  and  form  enzyms  needed  to  convert 
the  starch  and  other  insoluble  solids  into  soluble  ones. 
These  go  into  solution  in  the  cell-sap,  which  thereby  in- 
creases in  density  and  in  osmotic  potential.  The  cell-sap  in 
this  condition  rapidly  absorbs  water  and,  tending  to  in- 
crease proportionally  in  volume,  develops  a  pressure  equal 
to  the  force  needed  to  keep  it  at  that  volume.  This  pressure 
will  necessarily  be  exerted  upon  the  adjacent  cells,  will  thus 
be  extended  from  cell  to  cell,  the  substances  dissolved  and 
the  water  dissolving  them  will  pass  by  osmosis  in  the  same 
directions,  7.  e.  in  the  directions  of  least  resistance,  mechan- 
ical or  osmotic.  Thus  the  local  pressure  will  be  reduced  and 
the  danger  that  the  cells  may  burst  will  be  removed.  The 
total  pressure  may  remain  the  same,  or  the  local  pressure 
may  become  so  distributed  and  equalized  that  in  the  plant 
as  a  whole  there  will  be  none.  This  last  is  accomplished 
whenever  water  is  given  off,  as  vapor  or  liquid,  from  the 
surface  of  the  plant  in  amount  equal  to  that  absorbed. 

The  ratio  between  water  absorbed  and  water  given  off 
indicates  whether  there  can  be  any  pressure  of  the  cell-sap 
in  any  living  cell  (turgor-pressure)  or  in  the  whole  plant 
(sap-pressure).  The  greater  the  absorption  in  proportion 
to  the  loss  of  water  by  evaporation,  transpiration,  or  secre- 
tion, the  greater  the  pressure,  local  and  general ;  conversely, 


132  PLANT  PHYSIOLOGY 

the  greater  the  loss  of  water  in  proportion  to  the  absorp- 
tion the  lower  the  pressure,  local  and  general.  Copious 
absorption  is  dependent  upon  the  presence  in  the  cells  of  a 
large  amount  of  soluble  and  osmotically  active  substance 
and  upon  the  presence  outside  of  a  large  amount  of  water. 
Small  loss  of  water  is  dependent  upon  small  surface,  upon 
the  impermeability  of  the  walls  of  the  superficial  cells,  and 
upon  low  protoplasmic  activities  in  them.  These  condi- 
tions are  met  especially  in  spring,  but  also  to  a  limited  extent 
during  and  immediately  after  rain  in  summer.  In  spring, 
water  is  taken  up  in  quantity  from  the  moisture-laden  soil 
by  the  dense  cell-sap  of  the  root  cells;  from  the  still  bare 
branches  and  the  unopened  buds  water  is  given  off  only  in 
very  small  amount ;  *  sap-pressure  develops  in  consequence. 
If  a  plant  in  this  condition  has  been  so  recently  trimmed 
or  pruned  that  the  wounds  are  not  yet  closed,  or  if  new 
wounds  are  opened,  we  shall  have  the  familiar  phenomena 
of  " bleeding"  and  of  sap-flow.  The  name  "bleeding"  or 
"weeping"  is  given  to  wholly  useless  if  not  injurious  ex- 
hibitions of  the  phenomena,  employed  by  the  farmer  in 
northern  North  America  when  he  "taps"  his  maple  trees 
in  spring  to  secure  the  highly  prized  maple  syrup  and  maple 
sugar.  What  flows  from  the  plant,  whether  in  bleeding  or 
in  the  run  of  sap  after  tapping,  is  the  water  expressed  into 
the  wood  elements  by  the  living  cells  bordering  upon  them. 
This  sap,  flowing  out  under  pressure,  is  a  solution  contain- 
ing various  organic  compounds — in  maple  chiefly  sugars — 
and  mineral  salts.  The  presence  of  mineral  salts  in  the  sap, 
and  their  accumulation  in  the  evaporating  pans  employed  in 
sugar-making,  are  due  to  their  being  taken  up,  a  little  at  a 
time,  by  the  plant  in  the  water  absorbed  from  the  soil.  The 
amount  and  the  kinds  of  salts  present  in  the  sap  will  vary 
with  the  nature  of  the  soil  and  with  the  kind  of  plant,  for 
the  reasons  which  we  have  above  considered  (p.  112).  The 

*  It  is  not  a  question  of  surface  merely,  however,  for  in  evergreens, 
although  the  surface  is  not  materially  less  in  winter  and  spring  than  in 
summer,  the  amount  of  water  given  off  during  winter  and  spring  is  much 
less  than  in  summer.  The  lessened  activities  of  the  protoplasm  in  leaves 
and  branches,  and  the  decreased  evaporation  at  the  lower  temperatures, 
account  for  this. 


ABSORPTION  AND  MOVEMENT  OF  WATER  133 

amounts  and  the  kinds  of  organic  matters  will  vary  not 
only  with  these  two  factors,  but  also  with  the  condition  of 
the  individual,  both  at  the  time,  and  during  the  foregoing- 
season,  when  organic  substances  were  being  formed  and 
stored  by  the  plant. 

Sap-pressure,  which  determines  the  rate  of  sap-flow  and  of 
bleeding,  varies  at  different  times  in  the  season  and  in  the 
day.  The  amount  of  water  in  the  soil  and  of  moisture  in 
the  air  will  directly  affect  the  physical  conditions  of  sap- 
pressure  and  of  sap-flow.  If  the  soil  is  dry,  only  small 
amounts  of  water  can  be  absorbed,  the  turgor-pressure  in 
the  living  cells  of  the  root  will  be  relatively  low,  compara- 
tively little  water  will  be  pressed  from  these  cells  into  the 
wood-elements,  and  therefore  the  sap  in  the  wood  will  in- 
crease proportionally  little  in  volume  and  pressure.  If  the 
air  is  dry,  more  water  will  evaporate  from  the  plant,  the 
ratio  between  the  amounts  absorbed  and  given  off  will  be 
lowered,  and  the  volume  and  pressure  of  the  sap  in  the  wood 
will  be  proportionally  lowered. 

Other  factors,  acting  directly  upon  the  protoplasm  and 
only  by  this  means  affecting  the  physical  conditions  of  sap- 
pressure,  cause  the  pressure  and  rate  of  flow  to  vary  from 
time  to  time.  Whatever  stimulates  the  protoplasm  of  those 
cells  in  which  food  is  stored  in  solid  form  to  dissolve  the 
food,  will  tend,  other  things  being  equal,  to  raise  the  sap- 
pressure  by  increasing  the  absorption  of  water  and  the 
turgor-pressure.  The  increased  secretion  by  the  protoplasm 
of  diastatic  or  other  enzyms,  by  means  of  which  more  in- 
soluble solids  will  be  converted  into  osmotically  active  solu- 
ble substances,  increases  the  absorbing  power  of  the  cell-sap 
and  proportionally  increases  its  volume  and  pressure.  Con- 
versely, whatever  influences  depress  the  protoplasmic  activi- 
ties also  tend  to  reduce  the  sap-pressure. 

Experiments  by  Wieler*  on  plants  in  pots,  and  there- 
fore under  controllable  though  somewhat  artificial  con- 
ditions, confirm  the  observations  made  on  plants  in 

*  Wieler,  A.  L.  Das  Bluten  der  Pflanzen.  Cohn's  Beitrage  zur  Biologic 
der  Pflanzen,  Bd.  VI.,  1893.  Gain,  E.  Action  de  Peau  du  sol  sur  la  veg- 
etation. Revue  Generate  de  Botanique,  t.  VII.,  1895. 


134  PLANT  PHYSIOLOGY 

nature*  that  low  temperatures  (freezing  or  lower)  decrease 
the  sap-pressure  and  the  sap-flow  and  may  stop  the  bleeding. 
With  a  rise  in  temperature  the  pressure  will  rise  and  sap- 
flow  will  be  resumed.  Experience  shows  that  cold  at  night, 
stopping  the  sap-flow,  and  warmth  by  day,  causing  it  to  be 
renewed  with  vigor,  are  most  favorable  to  a  copious  yield 
of  sap  of  good  quality. 

The  means  of  measuring  the  pressure  developed  by  the 
absorption  of  water  are  at  the  best  inadequate,  for  the 
various  forms  of  pressure-gauges  (manometers)  employed 
cannot  be  made  to  measure  the  total  amount  of  force  de- 
veloped. A  manometer  measures  merely  the  net  force. 
Each  individual  cell  which  is  restrained  from  expanding,  and 
which  absorbs  more  water  than  it  gives  off,  exerts  force, 
develops  pressure.  But  by  no  means  all  the  cells  of  a  plant 
develop  pressure  simultaneously ;  some  cells  develop  no  more 
than  average  pressure,  and  some  dead  parts  (e.  g.  ducts, 
tracheids,  etc. )  cannot  develop  osmotic  pressure  under  the 
conditions  ordinarily  prevailing  in  the  plant.  Against  these 
less  resisting  cells  those  under  pressure  and  seeking  to  ex- 
pand, exert  force.  This  force,  being  partly  or  wholly  un- 
resisted,  expends  itself,  the  pressing  cells  expand,  the  others 
collapse.  Again,  water  may  be  forced  into  tracheids  and 
ducts  by  adjacent  cells  and  thus  the  pressure  of  the  latter  will 
be  reduced.  If,  however,  tracheids  and  ducts  become  filled 
with  sap,  as  is  the  case  in  early  spring  in  the  sugar  maple, 
vine,  etc.,  pressure  will  develop  in  these  dead  parts  also,  be- 
cause of  the  force  exerted  by  the  osmotically  active  living  cells, 
adjacent  or  more  or  less  remote.  The  pressure  developed  by 
one  group  of  cells  may,  then,  expend  itself  wholly  in  some 
other  part  of  the  body  of  the  plant,  leaving  no  force  to  be 
exerted  upon  the  pressure-gauge.  The  pressure-gauge  indi- 
cates only  that  amount  of  force  due  to  sap-pressure  which  is 
not  expended  in  the  body  of  the  plant  itself,  that  is,  the  net 
force  as  it  may  be  called,  to  distinguish  it  from  the  total  force. 

*  Bibliography  in  Wieler's  paper  above  and  papers  by  Jones  and  Orton, 
Sap-pressure  and  flow  in  Sugar  Maple.  Ann.  Report  Vermont  Agric. 
Exp.  Station,  1898.  Morse  and  Wood.  Studies  of  Maple  Sap.  Bulletins 
24,  25,  32,  New  Hampshire  Coll.  Agric.  Exp.  Station,  1895. 


ABSORPTION  AND  MOVEMENT  OF  WATER  135 

In  spite  of  the  inadequacy  of  the  means  for  measuring 
sap-pressure,  figures  of  very  considerable  magnitude  are  to 
be  found  in  the  published  studies  on  this  subject.  For  ex- 
ample— 

in  Ricinus  communis       6%  Ibs.  to  sq.  in. 
Urtica  dioica  9%     "     "         " 

Vitis  vinifera  21%     "     "         " 

Betula  alba  28        "     "         " 

Strangely  enough,  since  the  researches  of  Clark*  in  1873, 
little  attention  has  been  paid  in  this  country  to  the  phe- 
nomena of  bleeding,  hi  spite  of  the  facts  that  the  important 
maple-sugar  industry  depends  upon  it,  and  that  there  are 
botanists  at  the  Agricultural  Experiment  Stations  of  the 
sugar-making  States. 

For  the  very  natural  but  also  very  poor  reason  that  sap- 
pressure  has  often  been  measured  on  the  stumps  of  small 
plants  cut  off  near  the  ground— that  is,  where  the  sap-pres- 
sure must  develop  almost  wholly  within  the  root — it  has 
been  commonly  called  root-pressure.  That  this  is  a  mis- 
nomer follows  not  only  from  the  foregoing  consideration  of 
the  physics  of  sap-pressure,  but  also  from  the  introductory 
experiments  of  Pitra,f  repeated  and  extended  by  others, 
upon  the  sap-pressures  which  may  be  developed  in  parts 
above  ground,  for  example,  branches  cut  off  from  the  main 
stem  and  thus  wholly  separate  from  the  root.  The  sap- 
pressure  of  such  amputated  parts  may  even  be  higher  than 
of  those  left  attached  to  the  root.  It  is  true  that  the  sap- 
pressure  develops  first  in  the  lowest  parts  and  gradually 
ascends  the  stem  or  branch,  but  this  is  owing  to  the  absorb- 
ing part  being  below.  It  is  the  roots,  or  the  lower  ends  of 
amputated  branches,  which  absorb  the  water,  and  it  is 
necessarily  the  cells  nearest  the  absorbing  parts  that  first 
develop  pressure  and  from  which,  after  a  time  and  after  the 
pressure  goes  beyond  a  certain  height,  sap  is  pressed  out 

*  Clark,  W.  S.  Circulation  of  sap  in  plants.  Lecture  before  Mass.  State 
Board  of  Agriculture,  1874. 

t  Pitra,  A.  Versuche  iiber  die  Druckkraft  der  Stammorgane  bei  den 
Erscheinungen  des  Blutens  und  Thranens  der  Pflanzen.  Jahrb.  f.  w.  Bo- 
tanih.  Bd.  XI.,  1877. 


136  PLANT  PHYSIOLOGY 

into  the  tracheids  and  tracheae  to  make  room  for  the  water 
which  continues  to  be  absorbed.  Although  the  sap-pressure 
is  generally  greatest  at  the  base  of  a  stem  or  the  butt  of  a 
branch,  the  pressure  is  clearly  due  to  a  condition  in  some 
group  of  cells,  whether  these  are  in  the  root  or  elsewhere. 
We  should,  therefore,  abandon  the  misleading  name  of  root- 
pressure  and  use  only  the  equally  self-descriptive  but  correct 
term  sap-pressure. 

The  amount  of  sap-flow  is  exceedingly  different,  not  only 
for  different  species  and  different  individuals  of %  the  same 
species,  but  also  for  the  same  individuals  in  different  sea- 
sons. The  reasons  for  this  diversity  we  have  already  con- 
sidered. The  following  figures  will  serve  to  indicate  the 
volumes  sometimes  obtained — 

Birch  in  7  days  yielded  36        liters  sap    (Wieler) 

(12  years  old)         daily  average  5+ 

Agave  7.5          "       "      (Humboldt) 

(decapitated  flower  in  4.5  months  995.0      " 

Sugar  maple       in  23  days  93095.0  grs.  "      (Morse) 

(cuSference?;r"       daily  average  4047.0       »     « 

=3.6  litres  " 

(Sp.  gr.  1.32) 

TRANSPIRATION. 

From  all  their  surfaces  exposed  to  the  air,  plants  give  off 
water-vapor.  This  is  a  physical  necessity,  for  water-vapor 
will  be  given  off  from  any  mass,  lifeless  or  living,  which  con- 
tains water,  whenever  the  surrounding  air  is  not  saturated 
with  moisture,  or  when  the  mass  has  a  temperature  higher 
than  that  of  the  air,  or  when  the  mass,  in  relatively  dry 
air,  is  not  enclosed  in  a  waterproof  covering.  Other  things 
being  equal,  the  amount  of  water-vapor  given  off  will  be 
greater  the  greater  the  exposed  surface  in  proportion  to  the 
mass.  With  like  conditions  of  humidity,  temperature,  sur- 
face-composition, and  surface-area,  equal  masses  of  different 
composition  will  dry,  7.  e.  lose  water  by  evaporation,  at 
different  rates,  a  gelatinous  or  slimy  mass  more  slowly  than 
a  woody  one,  for  example.  The  living  plant  differs  from  a 
dead  one  of  exactly  the  same  dimensions  in  being  able  to 
control  four  of  these  five  factors,  and  to  that  degree  it  is 


ABSORPTION  AND  MOVEMENT  OF  WATER  137 

able  to  control  the  rate  and  the  amount  of  evaporation. 
Because  evaporation  from  the  body  of  the  living  plant  is 
controllable  within  certain  limits  by  the  plant  itself,  and  to 
this  extent  is  a  physiological  process,  it  has  been  given  the 
separate  name  of  transpiration.  With  this  idea  of  evapora- 
tion controlled  by  the  living  organism  has  been  coup- 
led the  notion  that  water  is  vaporized  by  the  chlorophyll 
grains  illuminated  by  sunlight  and  manufacturing  carbo- 
hydrates, and  that  this  water-vapor,  produced  by  physical 
means  acting  through  the  living  organs  of  the  plant,  is 
an  important  part  of  the  total  volume  of  water  given  off. 
This  process  Van  Tieghem*  distinguishes  from  evaporation 
by  the  name  of  chloro vaporization.  Assuming  that  water  is 
set  free  in  combining  carbon-dioxide  and  water  into  carbo- 
hydrate, it  is  hard  to  conceive  that  the  liberation  of  water 
in  a  water-containing  cell  would  be  in  the  form  of  escaping 
vapor  any  more  than  in  the  familiar  reactions  carried  on  in 
solutions  in  the  laboratory.  The  water  molecules  liberated 
at  the  temperatures  prevailing  in  cells  photosynthetically 
active  would  mix  with  the  water  in  the  cell-sap  both  in  the 
protoplasm  and  in  the  vacuoles.  Because  light  is  absorbed 
in  chlorophyll-containing  cells,  the  temperature  of  these  cells 
may  be  (but  not  necessarily  will  be)  higher  than  of  other 
cells  not  absorbing  light.  If  this  is  the  case,  and  if  these 
cells  are  warmer  than  the  air,  evaporation  will  of  course 
take  place.  Transpiration  is,  therefore,  a  physical  process 
controlled  but  not  carried  on  by  the  living  plant.  Accord- 
ing to  circumstances  it  may  be  more  or  less  rapid  than 
simple  evaporation. 

Plants  living  in  regions  where  the  rain-fall  is  slight  or  is 
very  unequally  distributed  through  the  year,  and  where  the 
soil  is  not  an  efficient  reservoir  of  water,  are  forced  not  only 
to  store  water  in  their  bodies  (see  p.  124)  but  also  to 
check  the  loss  of  water  by  every  possible  means.  The 
greatest  economy  of  water  is  shown  by  the  Cacti.  In  the 
most  condensed  forms  we  have  nearly  spherical  plants,  the 

*Van  Tieghem,  Ph.  Traite  de  Botanique,  t.  I.,  p.  185,  1891.  Also 
Transpiration  et  chlorovaporisation.  Bulletin  de  la  Soc.  Bot.  de  France, 

1886. 


138  PLANT  PHYSIOLOGY 

outer  surface  of  which  is  rendered  as  impermeable  as  possi- 
ble to  water  and  water-vapor  by  the  waxy  covering  of  the 
outer  walls  of  the  epidermal  cells,  by  greatly  thickening  and 
cutinizing  these  walls,  by  forming  more  than  one  layer  of 
epidermis,  by  sinking  the  guard-cells  of  the  stomata  to  the 
second  layer  of  epidermis,  by  greatly  reducing  the  number 
and  size  of  stomata,  and  in  some  cases  by  forming  a  woolly 
covering  of  dead  hairs  which  still  further  insulates  the  tissues 
within.  From  such  plants  the  loss  of  water  is  slight  in  pro- 
portion to  the  mass,  less  in  proportion  to  the  surface,  least 
in  proportion  to  the  amount  of  water  contained  in  .the 
plants.  Evaporation  from  these  plants  is  reduced  to  the 
minimum,  transpiration  is  lowest  in  rate  and  in  volume, 
first,  by  reason  of  the  composition  of  the  plant-body,  slimy 
and  gelatinous  materials  holding  water,  and  water-storing 
tissues,  forming  a  considerable  part  of  the  volume  of  the 
plant ;  second,  by  the  area,  composition  and  covering  of  the 
surface ;  third,  by  the  small  number  of  openings  through  the 
insulating  covering ;  and  fourth,  by  the  body-temperature  of 
the  plant  being  lower  than  that  of  the  air  when  the  air 
could  otherwise  take  up  most  moisture.* 

The  opposite  extreme  is  represented  by  plants  living  in  the 
very  humid  regions  of  the  tropics.  There  the  air  is  always 
near  the  point  of  saturation,  and  the  almost  daily  showers 
at  certain  seasons  attest  both  the  frequency  with  which  the 
air  attains  the  point  of  saturation  and  also  the  great 
amount  of  moisture  which  can  be  quickly  precipitated.  The 
well-known  copiousness  and  frequency  of  the  tropical  down- 
pours indicate  that  great  volumes  of  water  must  somehow 
be  vaporized.  It  has  sometimes  been  concluded,  from  the 
tardy  evaporation  of  water  from  wet  masses  having  the 
same  temperature  as  the  surrounding  air,  that  all  tropical 
plants  must  have  other  means  than  transpiration  for  get- 
ting rid  of  the  water  absorbed  by  them.  Some  do  have 
other  means  in  the  hydathodes  (p.  128),  but  not  many 

*  For  a  further  discussion  of  these  interesting  adaptations  consult 
Goebel,  Pflanzenbiologische  Schilderungen ;  Volkens.  Flora  der  agyptisch- 
arabischen  Wuste,  1887;  Schimper.  Pflanzengeographie  auf  physiolo- 
gischer  (jrundlage,  1898. 


ABSORPTION  AND  MOVEMENT  OF  WATER  139 

plants  are  so  equipped.  Whenever  the  temperature  of  the 
plant  is  higher  than  that  of  the  moisture-saturated  air 
outside,  transpiration  will  take  place,  the  water-vapor 
condensing  on  the  surfaces  of  the  plant  and  elsewhere. 
Because  the  plant  in  its  respiration  has  a  means  of  develop- 
ing heat,  it  must  often  be  the  case  in  the  tropics  that  the 
plant  is  warmer  than  the  surrounding  air.  That  transpira- 
tion into  the  moisture-laden  air  of  the  tropics  is  sufficient 
for  getting  rid  of  wrater  is  evident  when  we  take  into  con- 
sideration the  following  matters.  First,  the  plant  absorbs 
water  because  it  needs  and  uses  both  water  and  the  salts  dis- 
solved in  it.  Of  the  salts  it  needs  only  very  small  amounts, 
as  shown  by  culture  experiments,  and  though  analyses 
of  the  mature  plant  may  reveal  the  presence  of  much  larger 
amounts  of  some  or  all  of  the  useful  salts,  it  does  not  by 
any  means  follow  that  these  amounts  are  used,  or,  if  they 
are  used,  that  the  plant  is  not  over-fed.  Of  the  water  it 
needs  enough  to  bring  adequate  amounts  of  the  indispens- 
able salts,  and  if  the  plant  absorb  enough  water  for  this  pur- 
pose, it  will  certainly  have  sufficient  for  all  other  purposes 
also,  because  the  solutions  of  needed  salts  are  so  dilute. 
In  certain  places  one  of  the  advantages  attained  by  trans- 
piration certainly  consists  in  the  lower  body-temperature  of 
the  plant,  since  vaporization  is  a  cooling  process.  Second, 
transpiration  even  into  a  very  humid  atmosphere  will  suffi- 
ciently concentrate  the  cell-sap  of  superficial  cells  to  ensure 
an  osmotic  current  into  these  cells,  supplying  them  with 
both  the  water  and  the  salts  which  they  may  need.  Third, 
'no  more  water  will  be  absorbed  than  the  plant  needs,  for 
•  the  living  cells  control  by  their  activities  those  physical 
"conditions  which  make  absorption  possible.  If  the  trans- 
piration of  plants  living  in  the  humid  tropics  is  less  than  of 
plants  living  in  drier  regions,  the  absorption  of  water  will 
be  less.  So  long  as  water  and  needed  salts  are  absorbed 
in  sufficient  quantities,  growth  and  the  other  activities  of 
the  plant  will  be  normal.  The  luxuriant  vegetation  of  the 
tropics  impresses  every  one  with  the  idea  that  there  growth, 
food-manufacture,  etc.,  must  be  more  rapid  and  more  abun- 
dant than  in  temperate  regions.  The  correctness  of  this 


140  PLANT  PHYSIOLOGY 

view  is  questioned  by  Giltay,*  at  least  so  far  as  food- 
manufacture  is  concerned.  On  equally  fertile  soil  in  equal 
lengths  of  time,  the  activities  of  tropical  and  temperate 
plants  are  not  unlike.  If  this  is  true,  there  is  no  need  of 
more  rapid  absorption  and  transfer  of  food  in  tropical 
plants  than  in  those  of  temperate  climes,  and  transpiration 
may  at  least  be  no  more  rapid,  may  safely  be  less  rapid, 
than  in  dryer  temperate  regions.  It  may  easily  happen  in 
temperate  regions  that  the  plant  takes  in  more  water  and 
more  salts  than  it  really  needs,  and  that  while  the  former 
evaporates,  the  latter  accumulate  in  useless  forms  and 
quantities  with  or  without  chemical  change.  Whether  the 
transpiration  of  plants  adapted  to  the  climatic  conditions  of 
different  parts  of  the  world  differs  greatly  must  be  regarded 
as  still  unsettled',  f 

The  result  of  the  evaporation  of  water  from  any  solution 
is  the  concentration  of  the  solution  and  the  lowering  of 
its  temperature.  With  the  evaporation  or  transpiration  of 
water  from  the  leaves  of  plants,  the  cell-sap  of  the  cells 
giving  off  water  will  tend  to  increase  in  density  and  to  de- 
crease in  temperature  at  a  rate  proportional  to  the  rate  of 
transpiration.  The  increase  in  density  of  the  cell-sap  is  ac- 
companied by  an  increase  in  osmotic  pressure,  and  the  cell- 

*  Giltay,  E.  Uber  die  vegetabilische  Stoffbildung  in  den  Tropen  und  in 
Mitteleuropa.  Annales  du  Jardin  Botanique,  Buitenzorg,  t.  XV.,  1898. 

t  The  discussion  of  the  question  is  represented  by  the  following  papers  : 
Haberlandt,  G.  Anatomisch-physiolog.  Untersuchungen  iiber  das  tropische 
Laubblatt.  I.  Uber  die  Transpiration  einiger  Tropenpflanzen.  Sitzungs- 
ber.  d.  K.  K.  Akad.  d.  Wiss.  zu  Wien,  Bd.  CL,  Abth.  I.,  1892.  Uber  die 
Grosse  der  Transpiration  im  feuchten  Tropenklima.  Jahrb.  f.  wiss.  Bot- 
anik,  Bd.  31,  1898.  Erwiderung  zu  Giltay's  Abhandlung.  Jahrb.  f.  wiss. 
Bot.,  Bd.  33,  1899.  Stahl,  E.  Einige  Versuche  iiber  Transpiration  und 
Assimilation.  Botanische  Zeitung,  1894.  Burgerstein,  A.  fiber  die  Trans- 
pirationsgrosse  von  Pflanzen  feuchter  Tropengebiete.  Ber.  d.  Deutsch. 
Bot.  Gesellsch.,  1897.  Materialien  zu  einer  Monographic  betreffend  die 
Erscheinungen  der  Transpiration  der  Pflanzen.  Verhandl.  d.  K.  K.  Zool.- 
Bot.  Gesellsch.,  Wien,  1901  and  earlier.  Giltay,  E.  Vergleichende  Studien 
iiber  die  Starke  der  Transpiration  in  den  Tropen  und  im  mitteleuropai- 
schen  Klima.  Jahnb.  f.  wiss.  Botanik.,  Bd.  30,  1897.  Die  Transpiration 
in  den  Tropen  und  in  Mittel-Europa,  II.,  Jahrb.  f.  wiss.  Bot.,  Bd.  32,  1898 ; 
Ibid.,  III.,  loc.  cit.,  Bd.  34,  1900;  Ibid.,  Bot.  Centralbl.,  Beihefte,  Bd.  9, 
1900. 


ABSORPTION  AND  MOVEMENT  OF  WATER  141 

sap  draws  water  from  adjacent  cells  with  corresponding 
force.  The  decrease  in  the  water-content  of  the  parenchyma 
cells  causes  a  greater  draft  upon  the  water-conducting 
tissues  not  only  adjacent  but  throughout  the  conducting 
system,  for  a  draft  upon  one  part  disturbs  the  balance 
throughout  the  whole.  Thus,  assuming  an  adequate  force 
by  which  water  is  raised  through  the  ducts  from  root  to 
leaves,  we  see  that  this  force  is  set  in  motion,  and  its  action 
is  regulated,  by  the  amount  and  the  rate  of  transpiration. 
Transpiration  must  then  also  affect  the  root-hairs,  regulat- 
ing  the  amount  of  water  which  they  absorb.  A  current — for 
parts  of  its  course  osmotic,  for  the  remainder  of  much 
larger  dimensions— is  set  up,  maintained  and  controlled  by 
transpiration.  Transpiration  is,  however,  only  one  means 
of  doing  this,  water-pores  (p.  128)  and  hydathodes  (p.  128) 
being  the  others,  and  perhaps  equally  important  for  the 
plants  which  possess  them. 

The  ordinary  means  by  which  evaporation  is  controlled  by 
higher  plants  are  two:  fiist,  the  stomata  (p.  142),  which 
control  the  exchange  of  gases  as  well  as  of  water-vapor  be- 
tween the  plant  and  the  air ;  second,  the  character  of  the 
epidermal  and  other  cells  (cork,  etc.)  covering  the  plant.* 
Special  means  of  facilitating  or  checking  transpiration  are 
possessed  by  plants  inhabiting  especially  damp  or  especially 
dry  regions,  f 

*  Consult  De  Bary,  A.  Comparative  anatomy  of  the  vegetative  organs 
of  phanerogams  and  ferns.  Oxford,  1884. 

t  These  topics  need  not  be  discussed  here.  They  illustrate  no  new  princi- 
ples in  plant  physiology.  The  following  papers  may  be  read  by  the  inter- 
ested student.  Other  papers  and  books  have  been  referred  to  in  the  pre- 
ceding pages.  Stahl,  E.  Regenfall  und  Blattgestalt.  Ann.  du  Jardin  Bot. 
de  Buitenzorg,  Bd.  XI.,  1893.  Uber  bunte  Laubblatter.  Ibid.,  Bd.  XIII., 
1896.  Kny,  L.  Zur  physiologischen  Bedeutung  des  Anthocyans.  Atti  del 
Congresso  Botanico  Internationale,  1892.  (Older  literature  here  cited.) 
Keeble,  F.  W.  The  hanging  foliage  of  certain  tropical  trees.  Annals  of 
Botany,  vol.  IX.,  1895.  Darwin,  Charles  and  Francis.  The  Power  of  Move- 
ment in  Plants,  1880.  Stahl,  E.  Uber  den  Pflanzenschlaf  und  verwandte 
Erscheinungen.  Botanische  Zeitung,  I.  Abth.,  Heft.  V.,VI.,  1897.  Wilson, 
W.  P.  and  Greenman,  J.  M.  Preliminary  observations  on  the  movements 
of  the  leaves  of  Afelilotus  alba  L.  and  other  plants.  Contrib.  Botan.  Lab- 
oratory, Univ.  of  Pennsylvania,  1892. 


142  PLANT  PHYSIOLOGY 

STOMATA   AND  THE    AERATING  SYSTEM 

The  preceding  paragraph  leads  us  to  a  consideration  of 
those  special  epidermal  structures,  the  stomata,  which  are 
the  chief  means  by  which  the  plant  controls  the  transpira- 
tion of  water-vapor  and  the  exchange  of  oxygen  and  carbon- 
dioxide.  The  stomata  are  the  guarded  openings,  on  the 
surface  of  the  plant,  of  those  intercellular  spaces  which  form 
throughout  its  body  a  system  of  continuous  passages 
through  which  gases,  passing  diosmotically  into  and  out 
of  the  adjacent  living  cells,  make  their  way  from  and  to 
the  outside  air. 

Through  open  stomata  and  intercellular  passages  gases 
can  diffuse  more  rapidly  than  they  can  pass  by  osmosis 
through  cell-walls  soaked  with  water.  Besides  the  uninter- 
rupted diffusion  which  these  passages  and  openings  make 
possible,  still  more  rapid  movement  of  enclosed  gases  and 
vapors  must  take  place  whenever  the  plant  is  agitated, 
swayed  by  the  wind,  or  by  passing  animals.  Even  changes 
in  the  positions  of  the  organs  of  the  plant,  resulting  in 
changes  in  the  diameter  of  the  intercellular  spaces  in  one 
region  and  a  consequent  change  in  the  equilibrium  of  the 
enclosed  gases  and  vapors,  will  facilitate  the  movement  of 
gases  throughout  the  whole  aerating  system.  Thus  the 
constant  trembling  of  the  leaves  of  the  aspen  (Populus 
tremula,  and  P.  tremuloides),  and  often  of  other  plants 
also,  in  the  slightest  breeze,  and  the  autonomic  movements 
of  the  leaflets  of  Desmodium  gyrans  are  claimed  by  Stahl* 
to  increase  transpiration.  If  they  do  this,  they  must  also 
accelerate  the  interchange  of  gases. 

The  intercellular  spaces  which,  uniting  together,  form  with 
the  stomata  the  aerating  system  of  larger  massive  plants, 
.are  of  very  different  sizes  according  to  their  position  and 
according  to  the  needs  of  the  cells  enclosing  them.  Between 
the  chlorophyll-containing,  food-making  cells  of  the  leaf  the 
intercellular  spaces  are  comparatively  large.  In  the  leaves 
of  plants  growing  in  damp  places  they  are  even  larger  than 

*  Stahl,  E.    f  Tber  den  Pflanzenschlaf  und  verwandte  Erseheinungen.    Bot.  { 
Zeitung,  1897. 


ABSORPTION  AND  MOVEMENT  OF  WATER  143 

in  other  leaves.  *  The  large  size  is  due  to  the  need  of  get- 
ting rid  of  water-vapor  as  rapidly  as  possible,  and  of  ob- 
taining sufficient  quantities  of  the  carbon-dioxide  contained 
in  such  minute  proportions  in  the  air.  There  must,  there- 
fore, be  a  rapid  passage  of  comparatively  large  volumes  of 
air  past  these  mesophyll  cells.  On  the  other  hand,  the  in- 
tercellular spaces  in  the  deeper  tissues,  where  the  cells  de- 
mand mainly,  if  not  exclusively,  the  much  more  abundant 
oxygen,  are  relatively  very  small.  Merismatic  tissues — for 
example,  cambium— enclose  no  intercellular  spaces,  and 
though  aerated  only  osmotically,  they  obtain  oxygen  in  suf- 
cient  quantities  from  the  adjacent  tissues  which  do  enclose 
air  passages.  Hollow  stems— such  as  those  of  the  grasses 
— and  dead  cells  are  filled  with  air,  but  these  are  not  to  be 
regarded  as  forming  part  of  the  aerating  system.  On  the 
contrary,  they  merely  contribute,  like  the  air  chambers,  blad- 
ders, etc.,  of  water-plants,  to  the  lightness  of  the  organism. 
The  cells  enclosing  the  air  spaces  have  walls  which  are 
freely  permeable  to  water-vapor  and  to  gases.  Because 
these  cell- walls,  like  all  others  in  the  plant  except  those 
forming  the  outermost  bark-layers,  are  saturated  with 
water,  the.  passage  of  oxygen,  carbon-dioxide,  and  nitrogen 
(if  this  is  taken  up  at  all)  through  them  must  be  in  solu- 
tion, by  osmosis,  just  as  salts  enter  the  cells.  The  cell-walls 
bordering  upon  the  unconfined  air,  however,  are  so  modified 
chemically  by  cutinization,  suberization,  and  various  im- 
pregnations (e.  g.  with  SiO2),  and  by  being  overlaid  with 
wax,  that  they  are  far  less  permeable  to  gases  and  still  less 
to  water-vapor.  The  slow  exchange  of  gases  through  epi- 
dermis with  closed  stomata  or  with  none  has  been  re- 
peatedly demonstrated,  f  most  recently  and  certainly  most 
simply,  however,  by  the  use  of  dry  filter  paper  impregnated 
with  cobalt  chloride  and  by  the  iodine  test  for  starch. t 
It  is  well  known  that  cobalt  paper,  blue,  when  dry,  will 

*  Stahl,  E.  Einige  Versuche  iiber  Transpiration  und  Assimilation.  Botan- 
ische  Zeitung,  1894. 

tSee  discussions  of  this  in  Pfeffer,  Pflanzenphysiologie,  I.,  §§21,  29,  30. 
Engl.  transl.,  vol.  I.,  §§  21,  29,  30. 

t  Stahl,  E.    L.  c. 


144  PLANT  PHYSIOLOGY 

change  to  a  pink  hue  when  exposed  to  dampness.  This  fact 
has  been  employed  by  Stahl  in  demonstrating  that  transpira- 
tion through  the  walls  of  epidermal  cells  is  much  slower  than 
through  open  stomata.  The  iodine  test  for  starch  demon- 
strates whether  carbon-dioxide  enters  the  leaf  through  cell- 
walls  in  sufficient  quantities  for  starch  manufacture.  In 
most  plants  the  epidermal  walls  are  too  impermeable  for  this. 

Through  the  relatively  impermeable  superficial  tissues — 
epidermis  and  cork — there  must  evidently  be  openings  of  the 
intercellular  aerating  passages.  The  most  important  and 
the  most  perfect  of  these  openings  are  the  stomata,  found 
on  leaves  and  other  young  parts.  On  older  and  persisting 
parts  the  enclosing  cork  layer  may  be  interrupted  by  lenti- 
cels— masses  of  rounded  cells,  unchanged  as  to  their  walls, 
and  enclosing  intercellular  spaces  continuous  with  those 
deeper  in  the  body  of  the  plant.  Cork  may  be  replaced  on 
submersed  organs,  or  on  those  growing  in  the  mud  of 
swamps  and  marshes,  by  a  homologous  tissue,  like  that 
composing  the  lenticels,  and  called  a eren chyma.*  Lenticels 
can  be  closed,  and  the  passage  of  gases  through  aerenchyma 
can  be  stopped,  only  by  the  growth  of  new  tissue,  of  cork, 
which  seals  the  openings.  The  stomata,  on  the  contrary,  are 
intercellular  spaces  bounded  by  a  pair  of  delicately  balanced, 
and  in  most  cases  freely  movable,  epidermal  cells.  Lenticels 
are  found  especially  in  the  bark  of  stems  and  branches, 
though  also  on  older  bark-covered  roots.  Aerenchyma  is 
found  almost  exclusively  on  roots.  These  organs  secure 
mainly  the  entrance  of  oxygen  in  sufficient  quantities  into 
the  older  and  not  necessarily  very  active  tissues,  and  at  the 
same  time  the  exit  of  carbon-dioxide.  When  they  occur  on 
aerial  organs  they  of  course  facilitate  transpiration.  Sto- 
mata, on  the  contrary,  form  most  abundantly  in  the  epi- 
dermis immediately  covering  chlorophyll-containing  cells. 

Oxygen  forms  about  twenty  per  cent,  of  the  atmosphere 
and  carbon-dioxide  one-twentieth  of  one  per  cent.  The  chloro- 
phyll-containing cells  must  be  very  active  during  the  hours 
when  they  are  supplied  by  sunlight  with  the  energy  neces- 

*  Schenck,  H.  fiber  das  Aerenchym  :  ein  dem  Kork  homologes  Gebilde. 
Jahrb.  f.  wiss.  Botanik,  Bd.  XX.,  1889. 


ABSORPTION  AND  MOVEMENT  OF  WATER  145 

sary  for  photosynthesis.  Other  cells  can  work  at  a  slower 
rate  and  still  accomplish  as  much,  for,  being  independent  of 
light,  they  can  work  longer.  These  constitute  some  of  the 
reasons  for  the  position  and  for  the  number  of  stomata. 

Stomata  are  sometimes  called  " breathing  pores."  They 
do  admit  oxygen,  and  whenever  respiration  exceeds  photo- 
synthesis (e.  g.  in  the  dark),  the  unused  carbon-dioxide 
may  pass  out  through  them.  For  these  reasons  they  are 
breathing  pores,  but  this  is  neither  their  sole  nor  their  main 
function.  Through  open  stomata  water-vapor  passes  out- 
ward and  carbon-dioxide  inward.  Through  the  stomata  on 
the  surface  of  the  parts  of  the  flower,  little  carbon-dioxide 
passes  inward,  for  these  are  not  food-manufacturing  organs. 
On  the  contrary  water-vapor  and  expired  carbon-dioxide 
pass  outward.  On  such  organs  the  stomata  are  essentially 
organs  for  increasing  and  controlling  transpiration.* 

A  stoma  consists  fundamentally  of  a  pair  of  epidermal 
cells  not  completely  connected  together  and,  therefore,  either 
constantly  separated  from  each  other  by  a  slit-like  space,  or 
separated  whenever  the  cells  draw  or  are  drawn  apart.  The 
pair  of  incompletely  connected  cells — known  as  guard-cells — 
are  usually  strikingly  different  in  size,  form,  and  contents 
from  other  epidermal  cells.  The  cells  immediately  adjacent 
to  them  may  also  depart  from  the  type  of  epidermal  cells 
both  in  appearance  and  in  function.  Such  cells  when  present 
are  known  as  accessory  or  auxiliary  cells,  for  they  either  sup- 
plement the  guard-cells  in  their  task  of  opening  and  closing 
the  stoma,  or  they  themselves  open  and  close  the  stoma  by 
drawing  the  guard-cells  apart  or  pushing  them  together. 
Strictly  speaking,  the  stoma  is  the  slit-like  opening  (Spalt- 
offnung)  between  these  cells,  just  as  the  mouth  is  the  cavity 
opened  and  closed  by  the  lips. 

The  opening  and  closing  of  stomata  are  accomplished  by 
physical  means  only,  but  these  means  may  or  may  not  de- 
pend upon  the  irritability  of  the  living  protoplasm  of  the 
guard-cells  and  auxiliary  cells.  Fundamentally,  the  opening 

*  Chester,  Grace  D.   Bau  und  Function  der  Spaltoffnungen  auf  Blumen- 
blattern  und  Antheren.    Ber.  d.  Deutsch.  Bot.  Gesellseh.,  Bd.  XV..  1897. 
Older  literature  here  cited. 
10 


146  PLANT  PHYSIOLOGY 

and  closing  of  the  stomata  are  due  to  changes  in  the  tur- 
gor,* consequently  in  the  form  and  dimensions,  of  the 
guard-cells.  These  changes  in  turgor  may  be  due  wholly 
to  the  absorption  and  withdrawal  of  water  by  means  only 
remotely  if  at  all  controlled  by  the  living  cells,  or  they  may 
be  due  to  stimulations  of  the  living  cells  which  cause  them 
to  absorb  or  to  expel  water  or  otherwise  to  change  the 
density  of  their  cell-sap  with  consequent  changes  in  turgor. 
The  accompanying  figures  illustrate  a  stoma  without  aux- 
iliary cells  (Fig.  3)  and  with  auxiliary  cells  (Fig.  4). 
From  figures  5  and  6  the  differences  in  thickness  of  the 


FIG.  3.  FIG.  4. 

Figure  3.  Lower  epidermis  from  leaf  of  Vicia  faba — showing  stomata 
without  auxiliary  cells.  Figure  4.  Lower  epidermis  from  leaf  of  Tradescan- 
tia  zebrina— showing  stoma  with  auxiliary  cells. 

different  walls  of  the  guard-cells  are  evident.  The  thin  walls 
joining  the  guard-cells  to  their  neighbors  permit  rapid 
osmotic  transfer,  the  thin  walls  opposite  are  pliant.  If,  for 
any  reason,  the  guard-cells  absorb  water,  the  volume  of  the 
cell  cavities  will  increase,  the  form  of  the  cells  will  change, 
and  they  will  occupy  the  position  indicated  by  the  heavy 
outline  in  figure  5 ;  the  stoma  will  be  open.  If  the  reverse 
is  the  case,  if  the  amount  of  water  in  the  cell-sap  is  dimin- 
ished by  evaporation  or  expulsion,  the  volume  of  the  cell 
cavities  will  decrease,  the  thin  adjacent  walls  of  the  guard- 
cells  will  bend  out,  approach  each  other  and  finally  come 
into  close  contact;  the  stoma  will  be  closed.  From  the 
figures  it  will  be  noticed  that  the  guard-cells,  provided 

*  Mohl.  H.  von.  Welche  Ursachen  bedingen  die  Erweiterung  und  Vereo- 
gung  der  Spaltofmungen ?  Bot.  Zeitung.  1856. 


ABSORPTION  AND  MOVEMENT  OF  WATER 


147 


with  chlorophyll  grains,  are  capable  of  manufacturing  for 
themselves  substances  which  are  osmotically  active,  con- 
tributing to  the  turgor  as  well  as  to  the  nutrition  of  the 
cell.  The  other  epidermal  cells  of  most  plants,  being  devoid 
of  chlorophyll,  must  absorb  from  their  neighbors  the  osmoti- 
cally active  substances  upon  which  their  turgescence  depends. 
But  their  turgescence  will  vary  according  to  the  proportion 
sustained  between  the  amounts  of  water  which  they  absorb 
and  give  off.  If,  as  is  shown  in  figure  4,  the  shape,  size, 
position,  and  other  characters  of  some  or  all  of  the  epider- 
mal cells  adjoining  the  guard-cells  are  different  from  the 


Fig.  5.  Fig.  6 

Figure  5.  Diagram  (after  Schwendener)  to  illustrate  changes  in  form  of 
guard-cells  in  opening  and  closing  stomata.  Figure  6.  Cross  section  of 
stoma  of  Tradescantki  zebrinn,  showing  relations  of  auxiliary  cells  to  me- 
chanics of  opening  and  closing. 

other  epidermal  cells,  their  turgidity  will  vary  at  times,  at 
rates,  and  in  degrees  different  from  the  ordinary  epidermal 
cells.  With  these  changes  in  the  turgor  of  the  ordinary 
epidermal  cells,  of  the  auxiliary,  and  of  the  guard-cells,  there 
will  necessarily  be  changes  in  the  size  of  the  openings,  in  the 
degree  to  which  the  stcmata  are  open  or  closed.  As  re- 
cently re-stated  by  Darwin,*  in  opposition  to  the  extreme 
view  prevailing  of  late  that  the  guard-cells  alone  effect  the 
opening  and  closing  of  the  stomata,  "the  pressure  of  the 
guard-cells  and  that  of  the  surrounding  epidermis  should  be 
looked  at  as  correlated,  not  as  opposed  and  independent 


Darwin.   Francis.    Observations    on    Stomata. 
Society  of  London,  Series  B ,  vol.  190,  1898. 


Philos.  Trans.  Royal 


148  PLANT  PHYSIOLOGY 

factors."  Yet  it  is  easily  conceivable  owing  to  the  different 
rates  at  which  water  may  be  absorbed  or  given  off  by  the 
different  cells  of  the  epidermis,  that  in  some  plants,  or  under 
certain  conditions  of  culture,  etc.,  there  may  be  such  differ- 
ent conditions  in  the  epidermal  cells  that  while  the  guard- 
cells  may  be  expanding  and  so  tending  to  open  the  stoma, 
the  other  epidermal  cells  may  also  be  expanding  and  so 
tending  to  push  the  guard-cells  closer  together  and  to  close 
the  stoma.*  According  to  Benecke,f  the  auxiliary  cells 
assist  in  the  opening  and  closing  of  the  stomata  only  indi- 
rectly and  by  neutralizing  the  mechanical  strains  brought 
to  bear  on  the  guard-cells  by  the  changes  in  form  of  the 
epidermal  and  even  of  other  leaf  cells. 

Granting  that  changes  in  the  turgor  of  epidermal  cells, 
and  especially  of  those  peculiar  epidermal  structures,  the 
guard-cells,  bring  about  the  opening  and  closing  of  the 
stomata,  we  must  enquire  how  these  changes  are  effected 
and  what  are  the  results  as  regards  transpiration. 

Whenever  a  cell  loses  more  rapidly  than  it  absorbs  water, 
the  turgor  of  the  cell  will  decline  proportionally.  This  dif- 
ference will  occur  whenever  a  cell  is  unable  to  supply  itself, 
directly  or  through  its  neighbors,  with  water  enough  to 
make  good  the  loss  by  evaporation.  The  decrease  in  turgor 
of  the  guard-cells  of  the  stomata,  and  their  consequent 
flattening,  are  the  result  of  such  external  conditions;  the 
stomata  are  closed.  The  difference  in  the  amount  of  water- 
vapor  given  off  from  leaves  with  open  and  with  closed  stom- 
ata has  been  estimated  by  various  means.  The  most  striking 
way  of  demonstrating  this  is  perhaps  Stahl's  cobalt  paper 
method. t  Leaves  with  closed  and  with  open  stomata  are 
compared,  under  as  like  conditions  as  possible,  as  to  their 
rates  of  changing  the  color  of  dry  filter  paper  impregnated 
with  cobalt  chloride.  Whereas  the  sheet  of  cobalt  paper 
placed  against  the  leaf  with  open  stomata  will  change 

*  Stahl,  E.     I.e..  p.  121. 

f  Benecke,  W.  Die  Nebenzelleri  der  Spaltoffnungen.  Bot.  Zeitung,  p. 
602-3,  1892. 

t  Stahl,  E.  Einige  Versuche  tiber  Transpiration  und  Assimilation.  Bot. 
Zeitung,  1894. 


ABSORPTION  AND  MOVEMENT  OF  WATER  149 

within  a  few  seconds  from  blue  to  pink,  the  color  remains 
unchanged  for  an  hour  or  more  (with  Tradescantia  zebrina, 
for  four  hours)  in  the  paper  in  contact  with  the  closed 
stomata.  In  spite  of  the  very  minute  size  of  the  opening, 
even  at  its  fullest  extent,  the  great  number  of  stomata  give 
us  some  idea  of  the  effectiveness  of  these  gates  at  the  en- 
trances of  the  intercellular  passages.  Pfeffer,*  quoting 
figures  as  to  the  size  and  number  of  stomata,  says  the 
effective  area  of  the  opening  seldom  reaches  .0046  sq.  mm., 
but  that  the  number  of  such  openings  is  from  100  to  300 
per  sq.  mm.  Estimating  100  to  the  sq.  mm.  at  the  size 
.0046  sq.  mm.,  we  see  that  nearly  one-half  the  surface  can 
be  opened.  Assuming  that  the  stomata  are  on  one  side  of 
the  leaf  only,  and  that  on  that  side  they  have  the  propor- 
tions just  given,  we  see  that  roughly  one-quarter  of  the 
leaf-surface  can  be  the  free  path  for  the  exchange  of  gases 
and  vapors. 

The  disparity  between  absorption  and  evaporation  which 
for  physical  reasons  forces  the  stomata  to  close,  is  often 
supplemented  by  the  irritability  of  the  protoplasm  of  the 
guard-cells.  Evaporation  insufficient  to  produce  any  dimin- 
ution in  turgor  visible  as  wilting  may  still  be  enough  to 
irritate  the  g^ard-cells  into  reducing  their  turgor  by  physi- 
cal or  chemical  changes  accomplished  in  the  cells  by  the 
living  protoplasm.  Thus  various  influences  which  cannot 
bear  directly  upon  turgor,  but  which  can  irritate  the  proto- 
plasm, bring  about  the  closing  and  opening  of  the  stomata. 
The  statements  of  older  authors  regarding  these  influences 
have  been  lately  tested  by  Stahl,t  and  still  more  recently  by 
Francis  Darwin,!  from  whose  papers  the  following  conclu- 
sions are  abstracted.  The  stomata  of  most  plants  are  widest 
open  in  bright  light,  less  widely  open  or  completely  closed 
in  darkness.  This  one  would  in  general  expect,  for  photo- 
synthesis in  chlorophyll-containing  cells  ( and  the  guard-cells 
contain  chlorophyll)  is  most  rapid  in  bright  light.  There 

*  Pfeffer,  W.  Pflanzenphyeiologie,  I.,  p.  177.  Eng.  transl.,  I.,  p.  195. 
Compare  also  Brown  (Fixation  of  Carbon,  Nature,  1899),  and  pp.  45,  50 
of  this  book. 

f  7.  c.  J  1.  c. 


150  PLANT  PHYSIOLOGY 

would  then  be  the  greatest  demand  for  carbon-dioxide  to  be 
elaborated  into  food,  and  the  gateways  of  the  intercellular 
passages  should  be  open  to  allow  the  free  entrance  of  car- 
bon-dioxide. If  the  stomata  close  in  bright  light  to  guard 
against  excessive  transpiration,  food  production  diminishes 
greatly.  The  closing  of  stomata  more  or  less  completely 
in  darkness,  the  decrease  in  the  size  of  the  openings  as  the 
light  diminishes,  may  be  coupled  with  these  facts :  the  pro- 
portion of  oxygen  in  the  air  is  so  much  larger  than  that  of 
carbon-dioxide,  and  the  rate  of  respiration  so  much  lower 
than  that  of  photosynthesis  ( see  p.  65 ) ,  that  the  stomata 
may  well  decrease  in  size,  or  even  close  for  a  time,  with- 
out interfering  with  respiration ;  transpiration  on  dry  nights 
might  be  excessive  or  at  all  events  would  tend  to  cause  in 
the  cells  an  accumulation  of  salts  unnecessary  at  the  time 
in  amount  and  kinds;  the  leaf  is  cooled  by  transpiration, 
and  loss  of  heat  by  this  means  might  be  undesirable;  in 
nyctitropic  plants,  and  in  others  habitually  living  where  the 
air  is  damp,  the  closing  of  stomata  in  darkness  is  less  com- 
mon than  in  plants  growing  under  more  ordinary  condi- 
tions. Stomata  which  have  remained  closed  during  the 
night,  begin  to  open  at  daylight  in  the  morning.  Heat  tends 
to  open  the  stomata.  This  may  have  injurious  or  fatal  re- 
sults. If  the  leaves  of  an  evergreen,  for  example  of  holly, 
are  warmed  by  air  and  sun  while  the  roots  are  encased  in  soil 
so  cold  and  dry  that  the  root-hairs  absorb  too  little  water, 
the  opening  of  the  stomata  is  very  likely  to  be  followed  by 
excessive  transpiration,  by  the  drying  out  and  death  of  the 
plant.  Indeed,  most  cases  of  "  winter-killing"  are  to  be  at- 
tributed to  the  inability  of  the  plant  to  balance  transpira- 
tion by  absorption,  rather  than  to  actual  freezing  to  death. 
The  majority  of  plants  close  their  stomata  when  their 
leaves  are  wet.  This  can  be  demonstrated  by  putting  in 
water  strips  of  epidermis  from  the  leaves.  The  advantage 
is  more  obvious  than  the  means  by  which  it  is  attained.  If 
the  stomata  are  the  entrances  of  the  passages  through 
which  the  necessary  exchange  of  gases  and  vapor  takes 
place,  these  entrances  must  be  kept  free  from  whatever 
would  hinder  the  exchange  when  the  stomata  are  open. 


ABSORPTION  AND  MOVEMENT  OF  WATER  151 

Kain  and  dew  collecting  over  the  stomata  or  passing  into 
the  intercellular  spaces  would  prevent  the  diffusion  of  gases 
except  as  they  are  dissolved  hi  the  water.  This  disadvant- 
age is  avoided  in  the  first  place  by  placing  the  stomata  ordi- 
narily on  the  lower  side  of  the  leaf,  the  one  least  likely  to 
be  wet  by  rain.  It  is  further  avoided  by  the  fact  that  when 
the  leaf  is  wet,  transpiration  virtually  ceases  while  absorp- 
tion may  continue  for  a  time,  thus  producing  such  a  degree 
of  turgor  in  the  guard  and  other  epidermal  cells  that  their 
expansion  closes  the  stomata. 

Certain  plants  living  in  extremely  moist  places,  where  the 
danger  of  excessive  transpiration  at  any  season  is  reduced 
to  the  minimum,  are  unable  to  close  their  stomata.  Herbs, 
shrubs,  and  even  trees  (notably  the  willows)  possess  this 
character.  It  is  for  this  reason  that  these  plants  cannot 
bear  transplanting  to  dryer  places  for  cultivation,  but  are 
distinctly  swamp  plants. 

GASES   AND   THE   MOVEMENT   OF   GASES 

If  the  plant  or  any  part  of  it  dries— that  is,  loses  water 
faster  than  it  absorbs  it — air  takes  the  place  of  the  evapo- 
rated water.  If  the  cell-walls  and  intercellular  spaces  are 
equally  permeable  to  gases  and  to  water-vapor,  air  will  pass 
in  as  rapidly  as  evaporation  removes  the  water,  but  if 
water-vapor  makes  its  way  through  cellulose,  cuticula  and 
cork  faster  than  air,  drying  will  tend  to  produce  in  the 
plant  a  gas  pressure  lower  than  that  of  the  surrounding  air. 
If  again  the  component  gases  of  the  air  diffuse  and  diosmose 
at  different  rates,  the  evaporating  water  will  be  replaced  by 
nitrogen,  oxygen,  and  carbon-dioxide  in  proportions  "different 
from  those  hi  which  they  occur  hi  the  air.  But  if  the  living 
cells  of  the  plant  use  any  of  these  gases,  the  composition  of 
the  air  hi  the  plant  will  be  influenced  by  this  means  also. 
In  all  living  cells  of  higher  plants  under  normal  conditions 
oxygen  is  consumed  in  respiration  and  carbon-dioxide  is 
liberated,  usually  hi  equal  volumes.  In  all  chlorophyll-con- 
taining cells  carbon-dioxide  is  consumed  and  oxygen  lib- 
erated, under  the  influence  of  light.  Carbon-dioxide  diffuses 
and  diosmoses  more  rapidly  than  oxygen,  and  the  latter 


152  PLANT  PHYSIOLOGY 

more  rapidly  than  nitrogen,  but  these  gases  make  their  way 
less  rapidly  through  drying  cellulose  and  lignified  mem- 
branes, though  more  rapidly  through  cuticula  and  cork, 
than  does  water-vapor.  We  have  seen  that  the  wood,  which 
consists  mainly  of  the  walls  of  dead  cells,  is  the  path  of  the 
water-currents  from  root  to  leaves,  that  only  rarely  if  ever 
are  the  wood  elements  filled  with  water,  and  that  generally 
they  contain  air  and  water  in  alternating  columns  ( Jamin's 
chains ) .  Whenever  transpiration  is  more  rapid  than  water- 
transfer,  the  air-pressure  within  the  plant  will  decrease ;  there 
will  be  the  largest  volume  of  air  under  the  least  pressure 
when  transpiration  has  most  reduced  the  amount  of  water, 
and  when  the  vessels  are  fullest  of  water  there  will  be  the 
least  volume  of  air  under  a  pressure  equal  or  nearly  equal  to 
that  of  the  atmosphere.  The  constant  changes  in  the  rate 
of  transpiration,  causing  differences  in  the  water  content  of 
every  cell,  living  and  dead,  and  in  the  amount  of  water-vapor 
in  the  intercellular  spaces,  will  cause  constant  changes  in  the 
volume  and  pressure  of  the  gases  within  the  plant. 

More  strictly  vital  activities  also  affect  the  gas  pressures. 
As  may  most  conveniently  be  demonstrated  on  submersed 
aquatics,  photosynthesis  tends,  other  things  being  equal,  to 
produce  a  gas  pressure  within  the  plant  greater  than  that 
outside.  Because  the  stomata  of  a  land  plant  are  usually 
open  while  the  plant  is  manufacturing  food,  the  free  ex- 
change of  gases  between  the  plant  and  the  outside  air  keeps 
the  pressures  about  equal.  But  in  submersed  aquatics — e.  g. 
Elodea,  Ceratophyllum,  Myriophyllum,  etc. — the  gas  press- 
ure may  be  made  evident  in  two  ways.  In  the  first  place, 
the  buoyancy  of  the  plant  increases  while  it  is  illuminated, 
indicating  the  accumulation  of  gas  in  its  body ;  and  second, 
the  classical  experiment  of  inverting  and  cutting  or  prick- 
ing the  stem*  shows  that  a  stream  of  bubbles,  often 
countable  and  therefore  offering  an  index  of  the  photosyn- 
thetic  activity,  issues  from  the  cut  surface  or  from  the 
wound.  The  explanation  of  this  phenomenon  of  increased 
pressure  is  this  :  the  carbon-dioxide  used  in  the  elaboration 

*  For  description  see  Darwin  and  Acton's  or  Detmer-Moor's  laboratory 
manuals. 


ABSORPTION  AND  MOVEMENT  OF  WATER  153 

of  sugar  passes  by  osmosis  more  rapidly  into  the  body  and 
into  the  cells  of  the  plant  than  the  liberated  oxygen  passes 
out.  The  latter,  therefore,  accumulates  in  the  large  and  well 
developed  intercellular  spaces  and  passages,  escaping  only 
slowly  from  these  by  osmosis,  escaping  rapidly  only  when 
the  plant  is  wounded. 

The  reverse  of  this  result  is  attained  during  the  night, 
when  the  plant  is  photosynthetically  inactive  but  is  steadily 
respiring,  taking  in  oxygen  as  fast  as  it  needs  it,  and  giving 
out  the  more  rapidly  diosmosing  and  diffusing  carbon-di- 
oxide. But  since  respiration  is  never  so  active  as  photo- 
synthesis, the  negative  pressure  is  never  so  high  as  the 
positive. 

Since  plants  are  subjected  to  inconstant  but  frequent 
movement  by  the  winds,  by  passing  animals,  by  the  rise  and 
fall  of  the  tide,  by  waves,  etc.,  the  gases  contained  in  their 
bodies  are  subjected  to  varying  pressures,  are  forced  out 
and  drawn  in,  are  moved  from  part  to  part.  These  me- 
chanical influences  brought  to  bear  upon  plants  play  a  very 
important  role  in  contributing  to  the  movements  of  enclosed 
gases  and  vapors.  Besides  these,  temperature-changes  and, 
as  we  have  already  noted  in  connection  with  transpiration 
(p.  139),  the  temperature  of  the  plant-body  as  compared 
with  the  temperature  of  the  air  outside,  also  affect  the 
movements  of  gases  in  the  plant.  The  effects  of  these  in- 
fluences, however,  are  mainly  upon  the  gases  enclosed  hi  the 
intercellular  spaces  while  the  other  influences  which  we  have 
considered  affect  more  directly  the  gases  within  the  cells. 
But  all  of  these  influences  contribute  to  the  perfect  aeration 
and  ventilation  of  the  plant-body  just  as  the  elaborate 
musculature  of  the  higher  animal  automatically  maintains 
the  movements  of  the  gases  needed  by  the  cells  of  its  body. 
In  both  animals  and  plants,  osmosis  and  diffusion  underlie 
all  gas  movements  in  the  body,  but  both  are  controlled  by 
the  vital  activities,  that  is,  by  the  amounts  of  the  gases 
consumed  and  liberated  by  the  living  cells. 

From  the  foregoing  it  is  evident  that,  in  most  plants,  and 
under  ordinary  conditions,  the  composition  of  the  enclosed 
gases  can  differ  only  slightly  and  temporarily  from  that  of 


154  PLANT  PHYSIOLOGY 

the  surrounding  air.  In  certain  species,  gas  of  decidedly 
and  permanently  different  composition  from  that  of  the  sur- 
rounding air  accumulates  in  chambers  of  considerable  size. 
As  reported  in  a  preliminary  paper  by  Wille,  *  the  bladders 
of  the  Fucaceze  contain  absolutely  no  carbon-dioxide  and  a 
varying  percentage  of  oxygen,  thus— 

Bladders  wholly  immersed  in  water  contain  35-37%  oxygen 
lying  for  10  hours  in  air       "  20%       " 

darkened  for  12  hours  "  2.7%     (( 

These  figures,  in  connection  with  what  has  just  been  said 
about  the  different  diffusibilities  of  oxygen  and  carbon- 
dioxide,  are  significant.  The  buoyancy  of  the  plant  depends 
upon  its  photosynthetic  activity  and  upon  the  consequent 
accumulation  of  oxygen  which  is  collected  in  specially  differ- 
entiated reservoirs.  Nitrogen  necessarily  makes  up  the 
greater  part  of  the  total  volume  of  gas,  but  this  inert  gas 
varies  in  proportional  amount  only  because  of  the  produc- 
tion of  oxygen  in  photosynthesis  and  of  the  consumption  of 
oxygen  in  respiration  during  the  hours  of  darkness.  So  in 
all  plants  the  proportion  of  oxygen  in  the  intercellular 
spaces  decreases  in  darkness,  and  increases,  especially  in  the 
intercellular  spaces  of  photosynthetically  active  tissues,  in 
the  light.  The  impermeability  of  the  cutinized  membranes 
of  the  epidermal  cells  of  the  Fucacese  permits  the  develop- 
ment of  high  gas-pressure  by  means  of  the  abundant  pro- 
duction and  tardy  diffusion  of  oxygen.  Consequently,  in 
spite  of  their  large  intercellular  spaces,  these  plants  do  not 
collapse  even  in  very  deep  water,  f  Their  buoyancy  is  thus 
maintained  largely  by  gas-pressure,  and  though  the  form  of 
the  cells  is  maintained  by  their  own  turgescence,  the  form  of 

*  Wille,  N.  Abstract,  in  Just's  Jahresbericht  der  Botanik,  vol.  XVII.. 
p.  226,  1889,  of  a  "  vorlaufige  Mittheilung,"  in  Norwegian.  Also  Gasarten 
in  den  Blasen  der  Fucaceen.  Chem.  Centralbl.,  1890. 

f  Berthold,  G.  Uber  die  Vertheilung  der  Arten  im  Golf  von  Neapel. 
Mittheil.  a.  d.  Zoolog.  Station  in  Neapel,  Bd.  III.,  1882.  In  this  connec- 
tion it  should  be  mentioned  that  all  seaweeds  living  between  the  tide- 
marks  are  subjected  twice  daily  to  very  different  pressures.  Where  the 
range  of  the  tide  is  great,  as  in  the  Bay  of  Fundy  on  the  north  Atlantic 
coast,  the  algae  living  near  the  low-tide  mark  must  adapt  themselves  to 
the  great  differences  in  pressure. 


ABSORPTION  AND  MOVEMENT  OF  WATER  155 

the  whole  plant  is  due  to  the  high  gas-pressure  in  the  inter- 
cellular spaces  as  well  as  to  the  turgescence  and  form  of  its 
component  cells.  How  much  of  the  size,  and  to  a  certain 
extent  of  the  form  also,  of  plants  depends  upon  maintaining 
the  air-spaces  in  expanded  condition  can  only  be  roughly 
guessed  from  these  figures :  * 

|  to  £  of  the  volume  of  the  leaves  of  most  land  plants 
is  air-space. 

71%  of  the  volume  of  Pistia  texensis  (a  floating  plant)  is 
air-space. 

3.5%  of  the  volume  of  Begonia  hydrocotylifolia  (succulent) 
is  air-space. 

53%  of  the  volume  of  the  leaf  of  Polypodium  setigerum} 
is  air-space. 

TRANSLOCATION   OF   FOODS 

There  remains  for  us  to  consider  in  this  chapter  the  trans- 
location  of  foods.  Through  the  xylem  elements,  especially 
through  the  ducts  and  tracheids,  aqueous  solutions  of  cer- 
tain food-materials  are  transferred  from  the  absorbing  root- 
hairs  to  the  elaborating  chlorophyll-containing  parenchyma- 
cells  of  the  leaves.  To  these  same  chlorophyll-containing 
cells  the  other  food-material,  carbon-dioxide,  makes  its  way 
through  the  stomata  and  the  intercellular  spaces.  In  these 
cells  water  and  carbon-dioxide  are  consumed  in  elaborating 
a  carbohydrate,  a  food  which  accumulates  hi  the  same  or  in 
chemically  closely  related  form  in  the  cells  which  manufac- 
ture it.  From  these  cells  the  food  must  be  removed,  unless 
it  is  also  to  be  stored  there,  to  parts  needing  it  at  once  or 
,  in  which  it  can  be  kept  in  reserve  for  future  use. 

In  order  to  secure  the  transfer  of  the  non-nitrogenous 
food  manufactured  hi  the  green  tissues  during  the  hours  of 
daylight,  it  is  usually  necessary  to  change  the  chemical  con- 
stitution or  composition  of  the  food.  If  starch  or  oil  is 
the  form  in  which  the  carbohydrate  elaborated  by  the 

*  Pfeffer  W.  Pflanzenphysiologie.  I.,  p.  164.  Engl.  transl.,  I.,  p.  182. 
Various  papers  there  referred  to.  in  wMch  other  data  may  be  found. 

I  Estimated  from  Stahl's  figure  in  Botan.  Zeitung.  plate  IV.,  fig.  7,  1894. 
This  is  a  land  plant  from  one  of  the  rainiest  regions  in  the  tropics. 


156  PLANT  PHYSIOLOGY 


chlorophyll-granule  is  temporarily  deposited  in  the  cell,  the 
conversion  of  these  into  portable  compounds  is  advan- 
tageous. Although  oil  will  pass  through  cell-wall  and 
living  protoplasm,  the  same  amount  of  nutritious  material 
will  pass  through  the  same  distance  much  more  rapidly 
as  sugar  dissolved  in  water.  Starch,  being  insoluble, 
is  not  only  innutritious  as  such,  but  is  not  portable,  and 
hence  it  must  be  converted  into  a  soluble  substance,  also 
sugar.  What  is  true  of  the  non-nitrogenous  foods  elabo- 
rated in  those  organs  receiving  the  special  form  of  energy 
needed  for  their  manufacture,  is  also  true  of  the  nitrogenous 
foods,  elaborated  probably  in  all  the  living  cells  of  the 
plant,  whether  illuminated  or  not.  The  nitrogenous  foods, 
if  temporarily  deposited  in  insoluble  form  in  the  cells  elabo- 
rating them,  will  be  much  more  portable  if  converted  into 
soluble  forms.  As  we  have  already  seen,  the  non-nitro- 
genous foods  are  transferred  mainly  as  sugars,  the  nitro- 
genous mainly  as  amides. 

We  have  seen  (p.  116)  that  the  transfer  of  water  and 
dissolved  mineral  salts  from  the  absorbing  organs  to  those 
parts  in  which  they  are  used  as  food  or  from  which  water 
is  evaporated,  would  be  too  slow  to  secure  an  adequate 
supply  if  the  movement  were  wholly  osmotic.  In  order  to 
satisfy  the  needs  of  cells  consuming  food,  and  to  free  the 
manufacturing  cells  from  elaborated  food  so  that  they  may 
continue  to  make  it,  the  transfer  of  foods  must  also  be  by 
more  rapid  means  than  by  osmosis  merely.  In  small  and 
simple  plants  the  manufacture,  consumption,  and  storage  of 
food  may  go  on  in  the  same  cells  simultaneously.  In  larger 
plants,  division  of  labor  and  differentiation  of  tissues  secure 
greater  efficiency  and  economy.  In  these  plants  there  dif- 
ferentiates a  system  for  distributing  elaborated  foods  from 
the  places  of  manufacture.  The  earliest  and  simplest  con- 
ducting system  is  for  this  purpose.  We  find  such  a  system 
well  developed  in  the  large  marine  algae  (Fucus,  Laminarix, 
Nereocystis,  etc. ) .  When  plants  come  upon  the  land  it  be- 
comes hard  to  obtain  water  and  easy  to  lose  it.  When  plants, 
by  becoming  erect,  limit  their  water-absorbing  part  to  one 
end  and  place  their  food-manufacturing  cells  at  the  oppo- 


ABSORPTION  AND  MOVEMENT  OF  WATER  157 

site  end  of  the  body,  they  must^  supply  themselves  with  a  tis- 
sue system  for  the  rapid  conduction  of  water  and  salts  from 
roots  to  leaves.  The  wood,  which  carries  mainly  food  mate- 
rials, is  so  much  more  conspicuous  than  the  bast,  which  car- 
ries foods,  that  the  wood  seems  the  more  important  and  the 
earlier  needed  of  the  two.  This  may  be  true  of  land-plants, 
but  it  is  not  true  of  aquatics.  Phylogenetically  the  food- 
conducting  system  is  the  older;  in  aquatics  it  is  the  main 
or  only  tissue  for  the  rapid  transfer  of  aqueous  solutions. 

The  anatomical  distinctions  between  wood  and  bast 
are  so  evident  that  it  is  easy  to  infer  that  there  is  per- 
fect division  of  labor  between  these  two  sets  of  tissues. 
But  the  living  cells  of  the  bast  need  mineral  salts,  perhaps 
to  maintain  turgor  (p.  99),  to  neutralize  injurious  by- 
products (p.  100),  to  assist  in  the  construction  of  proto- 
plasm (p.  101) — and  will  receive,  transfer  and  use  them 
just  as  the  living  cells  of  the  wood  need  sugar  and  amides 
as  food  and  will  receive,  transfer,  and  consume  them. 

From  illuminated  chlorophyll-containing  cells  the  carbo- 
hydrates not  laid  down  in  solid  form  like  starch  or  in  slowly 
transferable  form  like  oil,  will  pass  by  exosmosis  to  adjacent 
cells  containing  less.  Such  osmotic  transfer  will  continue 
while  the  chloroplastids  are  manufacturing  diffusible  food 
and  until  osmotic  equilibrium  has  been  attained.  Osmotic 
transfer  will  begin  so  soon  as  the  carbohydrate  deposited  as 
starch  or  oil  is  converted  into  diffusible  form,  and  it  will  con- 
tinue so  long  as  starch  and  oil  are  converted  into  sugar. 
From  cell  to  cell  in  the  leaf  parenchyma,  and  from  this  into 
the  cortical  parenchyma  of  branch  and  stem,  osmotic  transfer 
will  take  place ;  but  it  will  take  place  always  most  rapidly  in 
the  direction  of  least  osmotic  pressure.  This  will  obviously  be 
toward  and  into  the  sieve-tubes  of  the  vascular  bundles.  The 
sieve-tubes — composed  of  comparatively  large,  long  cells  with 
thin  lateral  walls  and  perforated  cross-walls — are  continu- 
ous for  considerable  distances,  often  forming,  by  means  of 
anastamoses,  a  system  uninterrupted  from  tip  to  base  of  the 
plant.  In  the  sieve-tubes,  as  in  the  ducts  and  tracheids, 
there  will  tend  to  be  less  pressure  than  in  the  adjacent  cells. 
In  the  ducts  and  tracheids  the  danger  of  collapse  is  avoided 


158  PLANT  PHYSIOLOGY 

by  the  thickened  and  strengthened  walls.  In  the  sieve-tubes 
permanent  collapse  always  occurs  sooner  or  later  ( as  may 
be  seen  at  any  time  in  the  older  parts  of  the  phloem  in 
perennials  or  late  in  the  season  in  annuals ) .  It  may  be  pro- 
duced at  any  time  by  cutting  off  the  part— leaf,  branch,  or 
stem— whereupon  examination  will  reveal  the  sieve-tubes 
and  their  contents  in  the  abnormal  condition  figured  in  even 
the  most  recent  text-books.  Collapse  of  the  sieve-tubes  un- 
doubtedly occurs  more  or  less  completely,  though  only  tem- 
porarily, in  the  healthy  plant  whenever  the  leaf  or  branch  or 
stem  is  unduly  bent  and  whenever  the  removal  of  food  from 
the  sieve-tubes  by  the  adjacent  cells  for  use  or  for  storage  is 
more  rapid  than  the  supply  of  food  from  the  places  of  man- 
ufacture. The  pressure  in  the  sieve-tubes  will  vary,  just  as 
pressure  varies  in  any  cell,  according  to  the  prevailing  condi- 
tions; but  because  the  sieve-tubes  are  continuous  and  are 
in  contact  with  cells  which  consume  or  store  as  well  as  with 
cells  which  manufacture  food,  the  osmotic  and  other  pressures 
in  them  are  likely  to  be  lower  than  in  the  food-manufacturing 
cells.  Because  they  are  composed  of  long  cells,  the  cavities  of 
which  are  continuous  with  one  another  through  the  pores  in 
the  cross-walls,  the  sieve-tubes  are  especially  adapted  to  the 
translocation  of  the  osmotically  less  portable  nitrogenous 
foods,  especially  the  proteids.  It  would  appear  from  the 
investigations  of  Czapek*  that  it  is  especially  in  them,  not 
even  in  the  companion  and  cambiform  cells  of  the  phloem, 
that  the  diffusible  carbohydrates  also  are  transferred.  It 
does  not  necessarily  follow,  from  the  discovery  of  minute 
starch-grains  in  the  sieve-tubes,  that  they  pass  as  such 
through  the  sieve-like  cross- walls.  On  the  contrary,  starch 
occurring  in  the  sieve-tubes  must  be  regarded  as  carbohy- 
drate merely  temporarily  deposited  there  after  being  con- 
verted from  portable  to  stationary  form.  These  starch- 
grains  in  the  sieve-tubes,  like  the  great  majority  if  not  all 
of  the  solid  particles  in  other  cells,  are  too  large  to  pass 
through  pores  in  the  cross-walls.  \ 

*  Czapek,  F.    Zur  Physiologic  des  Leptoms  der  Angiospermen.    Ber.  d. 
Deutsch.  Bot.  Gesellsch..  Bd.  XV.    1897. 
f  Sachs,  J.  von.  Lectures  on  the  Physiology  of  Plants.  Engl.  trans. ,  p.  325. 


ABSORPTION  AND  MOVEMENT  OF  WATER  159 

The  presence  of  starch-grains  in  the  sieve-tubes  has  given 
occasion  to  the  hypothesis  that  either  the  sieve-tubes  them- 
selves, or  their  companion-cells,  are  the  chief  elaborators  of 
carbohydrates  into  nitrogenous  matters.*  Such  is,  how- 
ever, hardly  likely  to  be  the  fact  (p.  69 ).  Although  very 
likely  the  sieve-tubes  do  form  nitrogenous  food  from  nitrates 
and  carbohydrates,  this  function  seems  not  yet  to  have 
been  appropriated  by  any  special  tissue.  Apart  from  the 
classical  experiments  of  ringing  or  girdling  plants,  with  the 
result  that  the  downward  transfer  of  food  is  nearly  or  quite 
stopped,  it  must  be  admitted  that  exact  experiments  and 
definite  knowledge  on  the  special  functions  of  the  sieve-tubes 
remain  for  the  future,  t  The  recent  discovery  by  Raciborski  J 
of  a  substance  resembling  the  haemoglobin  of  higher  ani- 
mals in  that  it  readily  gives  up  oxygen  to  the  sieve-cells 
and  lactiferous  tubes,  in  which  it  regularly  occurs,  suggests 
that  besides  conducting  and  perhaps  contributing  to  the 
elaboration  of  foods,  these  tissues  may  also  obtain  needed 
oxygen  from  the  substances  which  they  conduct.  But  as 
Raciborski  says  in  his  second  paper,  this  discovery  made  in 
the  tropics  and  away  from  laboratories  must  be  tested  and 
made  the  starting-point  for  research  by  plant-physiologists 
with  laboratory  facilities. 

The  milk-tubes  or  lactiferous  ( laticiferous )  tubes  or  ves- 
sels, occurring  in  a  very  considerable  number  of  plants  and 
forming  continuous  systems  often  as  extensive  as  the  sieve- 
tubes,  are  filled  with  a  mixture  of  the  most  diverse  com- 
pounds dissolved  or  suspended  in  water.  Some  at  least  of 
these  substances  are  the  often  very  useful  by-products 
formed  in  nutrition,  respiration,  or  in  other  vital  processes. 
On  the  other  hand,  some  being  subjected  to  more  or  less 
profound  chemical  changes,  serve  as  sources  of  energy  in 
respiration,  as  materials  for  the  construction  of  cell- wall, 

*  Sachs.  J.  von.  Lectures  on  the  Physiology  of  Plants.  Engl.  transl.. 
p.  325. 

f  Trelease  (Sixth  Annual  Report.  Missouri  Bot.  Garden.  1894)  finds  no 
sieve-tubes  at  all  in  Leitnerin  floriflana. 

i  Raciborski.  M.  Ein  Inhaltskorper  des  Leptoms.  Ber.  d.  Deutsch.  Bot. 
Gesellsch..  Bd.  XVI..  1898.  Weitere  Mittheilungen  iiber  das  Leptomin.  ibid. 


160  PLANT  PHYSIOLOGY 

and  as  building-material  for  protoplasm.  Because  latex— 
the  contents  of  the  milk-tubes — is  composed  in  part  of  such 
nutritious  substances  as  starch,  sugars,  fats  and  oils,  and 
proteids,  and  because  the  milk-tubes  form  a  system  wholly 
uninterrupted  throughout  its  extent  by  cross-walls  of  any 
kind,  the  suspicion  is  irresistible  that  these  tubes  offer  the 
easiest  course  for  the  transfer  of  food  from  part  to  part. 
They  occur  in  plants  relatively  fewr  in  number  and  conse- 
quently cannot  be  indispensable  to  food  transfer.  Their 
functions  too  need  further  investigation. 

The  opposite  process  to  that  which  goes  on  in  the  chloro- 
phyll-containing cells  of  the  leaves,  whereby  the  elaborated 
carbohydrate  temporarily  deposited  as  starch  is  dissolved 
for  transport  elsewhere,  takes  place  in  the  organs  where 
carbohydrates  are  stored  in  solid  form.  In  roots,  rhizomes, 
and  tubers,  in  pith  and  medullary  rays,  and  in  seeds, 
parenchyma  cells  remove  the  sugar  from  the  solution  in 
which  it  comes  to  them  by  depositing  it  as  starch  grains  in 
the  protoplasm  or  as  cellulose  lining  their  walls.  How  the 
carbohydrates  are  acted  upon  by  the  protoplasm,  or  by  its 
special  organs  the  leucoplastids,  how  sugars  are  converted 
into  cellulose  or  into  starch  and  by  this  means  are  removed 
from  solution,  are  still  unanswered  questions.  It  is  easy  to 
see  that  if  sugar  is  removed  from  the  cell-sap  more  will  go 
by  osmosis  to  the  cells  removing  it  from  solution.  This 
last  is  a  necessary  physical  consequence  of  the  physiological 
(that  is,  in  this  case,  of  the  combined  chemical  and  physical) 
action  of  the  living  protoplasm.  But  this  storing  action  of 
the  protoplasm  is  as  little  understood  as  the  first  secretion 
of  sugar  in  nectaries  (p.  126). 

Still  less  comprehensible  at  the  present  time  are  the  ac- 
cumulations in  limited  regions  of  the  plant  of  substances 
which  ordinarily  would  move  osmotically  in  one  direction  as 
readily  as  in  another.  The  storage  of  carbohydrate  in  the 
iorm  of  inulin,  dissolved  in  the  cell-sap  of  dahlia-tubers  and 
in  the  underground  parts  of  other  Composite,  is  not  to  be 
explained  by  the  ordinary  laws  of  physics.  The  living  pro- 
toplasm which,  in  one  part  of  the  plant,  elaborates  food 
and  permits  its  exosmosis,  accumulates  food  and  prevents 


ABSORPTION  AND  MOVEMENT  OF  WATER  161 

its  exosmosis  in  another  part.  Similarly  in  the  filamentous 
alga,  the  accumulation  of  sugar  in  its  cells  bounded  on  all 
sides  by  membranes  permeable  by  water  is  conceivable  only 
on  the  hypothesis,  strengthened  by  analogy  ( p.  107 ) ,  that 
the  living  protoplasm  in  some  way  interferes,  either  physi- 
cally or  chemically,  with  the  exosmosis  of  the  sugar. 

To  summarize  the  results  of  the  discussions  in  this  chapter 
we  may  say  that  diffusion  and  osmosis  underlie  the  pro- 
cesses of  absorption  and  transfer  of  food-materials  and  of 
foods,  but  that  the  movements  of  these  gases  and  solutions, 
and  of  the  separate  substances  in  the  solutions,  are  con- 
trolled in  direction,  rate,  and  amount  by  the  living  proto- 
plasm. Its  needs,  and  the  amount  and  kind  of  work  it 
does,  change  or  establish  physical  conditions  and  set  in 
operation  physical  and  chemical  laws. 


CHAPTER   V 
GROWTH 

The  processes  so  far  discussed  supply  the  plant  with  the 
materials  and  with  the  energy  to  carry  on  other  processes 
popularly  regarded  as  functions  peculiar  to  living  beings, 
and  at  all  events  performed  with  a  greater  degree  of  inde- 
pendence and  self-control  by  them  than  by  lifeless  objects. 
In  this  respect,  however,  these  processes  do  not  differ  from 
those  already  examined,  for  we  have  seen  that  respiration 
is  oxidation  controlled  and,  to  a  certain  extent,  carried  on 
by  the  living  organism ;  that  nutrition  depends  upon  chemi- 
cal syntheses  accomplished  by  living  protoplasm,  which  uses 
simple  substances  obtained  by  itself,  through  its  application 
of  the  laws  of  diffusion  and  osmosis;  that  the  absorption 
and  transfer  of  food-materials  and  of  foods  are  physical 
processes  made  possible  and  regulated  by  the  living  organ- 
ism. The  processes  which  we  have  still  to  study — growth, 
irritability,  and  reproduction — involve  and  consist  in  chemi- 
cal changes  not  confined  to  living  organisms  but  controlled 
by  them. 

Growth  cannot  be  understood  unless  the  sensibility  of  the 
growing  substance  is  constantly  borne  in  mind.  A  crystal 
of  copper  sulphate  in  a  concentrated  and  slowly  evaporating 
solution  of  this  salt  could  not  grow  if  the  molecules  of  cop- 
per sulphate  were  not  mutually  attractive,  if  those  which 
were  still  moving  freely  in  the  solution  did  not  respond  to 
the  attraction  exerted  upon  them  by  those  already  settled. 
The  limits  of  form  and  size,  of  rate  and  direction  of  growth, 
of  copper  sulphate  crystals,  are  fixed  by  physical  laws,  but 
though  these  laws  are  universal,  the  balance  in  any  one 
spot  may  be  very  different  from  that  elsewhere,  and  the 


GROWTH  163 

growing  crystal  will  conform  and  must  conform  to  its  en- 
vironment, It  is  determined  by  physical  law  that  copper 
sulphate  molecules  will  not  arrange  themselves  in  the  crys- 
talline form  without  a  definite  amount  of  water;  if  the 
amount  of  water  be  excessive  or  deficient  the  copper  sulphate 
molecules,  having  always  the  same  attraction  for  one  an- 
other, will  still  be  unable  to  obey  it,  to  approach  one  an- 
other, and  to  arrange  themselves  in  their  due  order.  The 
crystals  of  the  same  substance  will  be  small  or  large  accord- 
ing as  they  are  formed  fast  or  slowly.  And  what  is  true  of 
the  behavior  of  one  substance  alone  in  solution  in  another 
is  also  true  of  many  substances  together  in  solution, 
whether  these  find  themselves  in  a  cell  or  wholly  outside  a 
living  body.  The  molecules  of  the  different  substances  will 
mutually  attract  or  repel  one  another,  they  will  be  indiffer- 
ent or  they  will  decompose  one  another,  and  when  a  state 
of  balance  is  attained,  the  molecules  which  result  from  all 
the  changes  wrill  arrange  themselves  in  their  characteristic 
ways.  Because  the  living  protoplasm  itself  is  composed  of 
molecules  forming  a  definite  structure,  this  structure,  like 
the  whole  crystal,  is  subject  to  physical  influences,  and  its 
component  molecules  are  obedient  to  physical  laws.  So  the 
molecules  and  groups  of  molecules  forming  the  living  proto- 
plasmic structure  are  pulled  down  by  gravitation.  The 
molecules  vibrate  with  lesser  amplitude  and  draw  together 
in  cold,  vibrating  with  greater  amplitude  and  so  tending  to 
move  apart  when  warmed.  And  as  warmth  makes  molecu- 
lar movements  in  any  substance  freer,  so  water  makes  pos- 
sible still  greater  freedom  of  molecular  movement  in  those 
substances  which  dissolve  in  it.  There  are  substances,  how- 
ever, which  neither  repel  water  as  do  the  fats  and  oils,  nor  go 
into  solution  in  it  as  do  many  salts,  but  which  still  absorb 
water  in  great  quantity.  This  absorption  results  in  greater 
freedom  of  molecular  and  even  of  massive  movement.  Pro- 
toplasm is  one  of  these  substances;  it  swells  as  it  absorbs 
water,  and  its  circulation,  as  well  as  its  molecular  move- 
ments, becomes  more  rapid,  up  to  the  optimum.  If  still 
more  water  is  forced  into  it,  the  molecules  and  groups  of 
molecules  composing  it  will  be  forced  so  far  apart  by  the 


164  PLANT  PHYSIOLOGY 

molecules  of  water  enclosed,  that  the  definite  protoplasmic 
;  structure  will  be  strained  or  destroyed,  and  the  massive 
movements  will  accordingly  change  or  cease. 

Without  going  further  into  the  subject  now  (see  the  next 
chapter),  it  may  be  stated  that  the  growth  of  the  living- 
organism,  like  that  of  the  crystal,  is  in  accordance  with  the 
sensibility  of  its  component  molecules  and  groups  of  mole- 
cules to  physical  and  chemical  influences.  The  difference 
between  the  lifeless  crystal  arid  the  living  organism  can  be 
suggested — not  definitely  stated,  however — in  this  way  :  the 
molecules  of  the  crystal,  and  the  crystal  as  a  whole,  are 
entirely  subject  to  the  prevailing  balance  of  physical  forces ; 
the  living  organism,  on  the  contrary,  is  able  to  modifj^, 
change,  or  maintain  the  balance  of  physical  forces.  When  it 
ceases  to  be  able  to  do  this  it  dies,  it  becomes  like  the 
crystal.  This  power  peculiar  to  living  organisms,  whatever 
it  may  be  dependent  upon,  does  not  necessarily  imply  any 
greater  sensitiveness  to  physical  forces  than  would  be  repre- 
sentable  by  the  sum  of  the  sensibilities  of  all  the  substances 
composing  and  contained  within  the  living  organism  (p. 
185).  But  the  organism  is  sensitive,  and  its  growth  cor- 
responds to  the  forces  or  influences  to  which  the  organism 
is  subjected  quite  as  much  as  to  the  matter  with  which  it 
is  supplied. 

What  is  growth?  Of  this  no  adequate  definition  can  be 
given,  although  for  each  mind  the  word  possesses  a' certain 
significance.  Physiologists  have  always  attempted  to  state 
in  what  growth  consists,  and  have  always  failed.  Our  con- 
ception of  growth  should  be  clarified  at  once  by  distinguish- 
ing this  phenomenon  from  the  others  ordinarily  accompany- 
ing it.  Growth  is  a  part  of  the  process  of  development. 
Growth  and  differentiation  together  accomplish  develop- 
ment. Differentiation  is  that  specialization  in  structure 
which  follows  and  contributes  to  specialization  in  function. 
Differentiation  is  limited  by  the  number  of  parts  and  of 
cells,  in  other  words,  by  the  size  of  the  body.  In  a  small 
body  little  differentiation  is  possible,  in*  a  large  one  much. 
But  it  does  not  follow  from  this" that  the  largest  bodies  are 
the  most  highly  differentiated  in  structure  and  function; 


GROWTH  165 

they  are  not  yet,  nor  will  they  ever  be  unless  differentia- 
tion keeps  pace  with  growth.  Does  growth  then  consist 
in  increase  in  size?  A  cell  may  elongate,  but  this  will 
not  necessarily  be  growth.  We  can  easily  imagine  a  cell 
being  subjected  to  a  pull  which  would  elongate  it,  or  to 
a  pressure  which  would  extend  as  well  as  compress  it;  or, 
indeed,  within  the  cell  itself  a  pressure  resisted  less  in  one 
direction  than  in  others  would  cause  the  cell  to  elongate. 
In  the  last  case  there  is  stretching  because  of  the  pressure 
developed  within  the  cell  by  its  osmotically  active  contents. 
In  the  two  preceding  cases,  the  cell  changes  its  dimensions 
because  it  is  subjected  to  a  force  from  outside  itself  which 
it  cannot  resist.  In  none  of  these  cases  has  growth  taken 
place.  If,  however,  these  changes  in  dimensions  are  made 
permanent  by  work  done  by  the  living  cell  itself  in  translo- 
cating and  depositing  insoluble  material  in  such  positions 
as  to  fix  the  new  form  or  the  new  dimensions,  growth  will 
have  taken  place.  Growth,  the  fixing  of  the  new  form  or 
new  dimensions,  follows  the  change  accomplished  from  with- 
out. But  permanent  change  of  form  or  of  dimensions  may 
be  accomplished  by  forces  wholly  outside  the  plant,;  for 
example,  by  stretching  the  cell  beyond  the  limit  of  elasticity 
of  its  walls.  This  would  not  constitute  growth. 

Growth,  then,  does  not  necessarily  consist  in  an  increase 
in  volume,  for  there  are  evidently  cases  of  unquestioned 
growth  without  this.  The  boy  increasing  in  stature  and  the 
vine  increasing  in  length,  decreasing  in  breadth  or  thickness 
meanwhile,  are  growing  though  not  increasing  in  volume. 
There  is  necessarily  increase  in  volume  of  parts  or  organs, 
but  not  of  the  whole  organism,  in  such  cases  of  growth. 
On  the  other  hand  a  rise  in  turgor  which  increases  the 
volume  of  a  part  is  not  growth ;  this  is  merely  expansion. 
Growth  is  a  process  dependent  upon  the  formation  of  new 
protoplasm,  and  though  it  usually  results  in  increase  in 
volume,  in  increase  in  weight  or  mass,  and  in  increase  in 
substance,  it  is  not  essentially  any  of  these. 

Growth  is  made  possible  by  cell-division,  but  it  does  not 
consist  in  the  formation  of  new  cells,  for  new  cells  can  be 
formed  by  the  mere  division  of  old  cells.  Each  cell,  each 


166  PLANT  PHYSIOLOGY 

kind  of  cell,  and  hence  each  organism,  has  a  maximum 
which  its  size  normally  never  exceeds.  Cells  which  have  at- 
tained their  maximum  size  can  continue  to  contribute  to 
the  growth  of  an  organ  only  after  doing  what  will  make 
possible  the  formation  of  new  protoplasm.  Cell-division,  a 
process  of  rejuvenation,  accomplishes  this.  The  smaller 
daughter  cells,  each  with  its  own  nucleus  instead  of  with 
only  a  share  of  the  single  nucleus  of  the  undivided  cell,  are 
more  vigorous  than  the  mother-cell  at  maturity ;  they  form 
new  protoplasm  as  well  as  other  products  from  the  food 
furnished  them ;  and  they  then  increase  in  size.  In  a  meris- 
tematic  tissue,  such  as  that  at  the  tip  of  the  stem  or  root 
or  in  the  cambium,  we  have  the  first  and  the  fundamental 
stage  in  the  process  of  growth,  namely,  the  formation  of  new 
protoplasm  and  of  new  cells.  Behind  the  tip  of  stem  and 
root,  and  on  either  side  of  the  cambium,  we  have  the  later, 
the  evident  stage,  when  increase  in  size  or  volume,  in  mass 
and  in  substance,  takes  place. 

Evident  growth  takes  place  when  the  body  or  any  part  of 
it  permanently  increases  in  volume  or  in  size.  Increase  in 
substance,  which  results  in  increase  in  weight,  may  take 
place  after  all  growth  has  ceased.  It  may  be  merely  the 
storing  up  of  food  elaborated  at  one  time  to  be  used  later, 
or  it  may  be  the  absorption  of  water.  This  last,  however, 
invariably  results  in  the  increase  in  volume  of  the  parts, 
or  in  the  increase  in  turgor,  or  both  at  once.  Any  ab- 
sorption of  water  by  dry  animal  and  vegetable  substances 
is  followed  immediately  by  swelling — a  phenomenon  from 
which  hypotheses  regarding  the  minute  structure  of  organ- 
ized bodies  have  been  more  or  less  successfully  deduced. 
Beyond  the  point  at  which  swelling  by  the  intussusception 
of  molecules  of  water  between  the  molecules  or  groups  of 
molecules  of  the  formed  substances  would  cease,  increase  in 
volume  may  still  go,  by  reason  of  the  structure  and  compo- 
sition of  cells.  The  presence  of  osmotically  active  substances 
in  cells  which  can  absorb  water  ensures  increase  in  pressure 
within  the  cells,  and  this  pressure  will  distend  the  enclosing- 
protoplasmic  and  cellulose  or  other  walls.  Up  to  a  certain 
point,  easily  conceived  but  not  easily  determined,  the  in- 


GROWTH  167 

crease  in  volume  due  to  the  absorption  of  water  is  genuine 
growth,  and  the  second  stage  in  growth,  the  one  which  I 
have  designated  as  evident  growth,  consists  mainly  if  not 
wholly  in  the  absorption  of  water.  But  as  the  water-con- 
tent of  any  cell  or  tissue  or  organ  is  subject  to  fluctua- 
tion, and  as  the  turgor  and,  consequently,  the  volume 
also  fluctuate  with  the  water-content,  it  is  difficult  to  tell 
when  growth  ceases  and  turgor-swelling  begins.  Both  are 
vital  processes  in  the  sense  that  they  depend  upon  the 
physical  and  chemical  conditions  established  by  the  living 
protoplasm,  but  they  are  distinct  processes,  fluctuations  in 
size  due  to  changes  in  turgor  taking  place  even  long  after 
true  growth  has  ceased  to  be  possible  (see  pp.  167,  168). 
Whether  growth  depends  upon  turgor  or  vice  versa  is  still 
to  be  conclusively  shown  by  experiment. 

That  evident  growth  consists  mainly  in  the  increase  of  the 
water-content  of  the  growing  part  has  long  been  known  to 
be  the  case  in  plants,  though  only  recently  properly  em- 
phasized for  animals.  *  A  longitudinal  section  through  the 
tip  of  a  growing  stem  or  root,  or  a  cross-section  through 
the  stem  of  a  dicotyledonous  plant  during  its  period  of 
growth  in  diameter,  wrill  show  at  least  three  distinguishable 
regions.  These  are  indicated  in  the  accompanying  figures  ( pp. 
168, 169 )  of  a  longitudinally  sectioned  root  of  Azolla.  In  the 
diagramatic  Figure  .7  the  three  regions  of  cell-formation  ( 1 ) , 
cell-growth  (2),  and  cell-differentiation  (3),  are  indicated. 
These  are  shown  in  detail  in  the  figures  8,  9,  10.  Figure  8 
(corresponding  to  1  in  7)  represents  the  tip  of  the  root 
with  its  cap  (Cap),  dermatogen  and  epidermis  (Ep.), 
cortex  (Cor. ),  and  central  cylinder  (c.  c. ),  aU  of  which 
come  directly  or  indirectly  from  the  division  of  the  large 
apical  cell.  The  meristematic  and  embryonic  cells  are  full  of 
dense  protoplasm.  Figure  9,  taken  from  further  up  in  the 
same  root,  corresponds  to  region  2  in  figure  7,  and  shows 
that  the  increase  in  size  of  the  cells  of  the  different  layers  is 
accompanied  by  a  great  increase  in  the  volume  of  the  cell- 

*  Davenport,  C.  B.  The  role  of  water  in  growth.  Proc.  Boston  Soc. 
Nat.  History,  vol.  28,  1897.  Experimental  Morphology.  Part  II..  1899. 
and  the  literature  there  cited. 


168 


PLANT  PHYSIOLOGY 


sap,  which  accumulates  in  large  vacuoles,  without  there  being 
any  considerable  increase  in  the  amount  of  protoplasm  in 
each  cell.  Figure  10  corresponds  to  region  3  of  figure  7,  and 
shows  how  the  cells  change  in  taking  on  their  definitive 
characters,  Ves.  indicating  some  of  the  changes  taking  place 
during  the  formation  of  a  vessel  in  the  central  vascular 
bundle.  In  this  region  the  amount  of  protoplasm  decreases 


Ep.  Cor.  Ves. 

FIG.  7.  FIG.  9.  FIG.  10. 

Figures  7—10.  Longitudinal  sections  of  Azolla  root.  Fig.  7— diagram- 
atic— showing  region  of  cell-formation  (1).  cell-growth  (2).  cell-differenti- 
ation (3).  Fig.  9.  =  2  in  Fig.  7  more  highly  magnified.  Fig.  10.  =  3  in 
Fig.  7  more  highly  magnified. 

not  only  proportionally  but  absolutely.  There  can  be  little 
more  if  any  permanent  increase  in  volume  in  this  region, 
although  there  may  be  increase  as  well  as  decrease  in  vol- 
ume because  of  differences  in  turgor  solely. 

The  factors  contributing  to  make  growth  possible  may  be 
grouped  under  three  heads  :  1st,  there  must  be  an  adequate 
supply  of  material ;  2d,  there  must  be  an  adequate  amount 
of  room ;  3d,  there  must  be  the  impulse.  Physiologists  are 
not  able  to  reduce  to  definite  physical  and  chemical  terms 
what  is  comprehended  under  this  last  head,  and  it  has  still 


GROWTH 


169 


to  be  ascertained  whether  the  necessary  impulse  comes  from 
within  or  from  without,  whether  it  is  inherited  or  is  new. 
But  without  the  impulse  there  will  be  no  growth. 

Growth  will  not  be  possible  without  the  needed  materials 
and  space.  The  substances  essential  for  growth  are  those 
essential  for  life,  but  they  may  be  grouped  into  two  cate- 
gories— nutritious  substances,  and  otherwise  useful  sub- 
stances. The  nutritious  substances  furnish  the  materials  of 
which  the  protoplasmic  structure  and  the  cell- wall  are  built, 
and  those  compounds  which  in  respiration  yield  the  energy 
needed  by  the  part  to  complete  the  first  stage  of  growth. 

If  the  supply  of  food  is  constantly  sufficient  during  the 
period  of  growth,  both  construction  and  enlargement 


c.c 


Cor 


FIG.  8. 


Figure  S.    Tip  of  root  of  Azolki  showing  apical  cell  and  region  of  cell- 
formation  =1  in  Fig.  7  more  highly  magnified. 

will  be  uniform,  other  things  being  equal;  but  if  the  sup- 
ply varies  in  amount,  the  rate  of  growth  will  vary  cor- 
respondingly. It  is  found,  for  example,  that  the  growth  of 
green  and  independent  plants  is  periodic  in  a  much  more 
marked  degree  than  that  of  plants  which  obtain  their  food 
ready-made.  During  the  day,  while  food  is  being  made 
and  accumulated  in  the  organs  photosynthetically  active, 


170  PLANT  PHYSIOLOGY 

growth  is  slower  than  during  the  night,  when  food  is  sup- 
plied in  abundance  to  the  growing  parts.  *  This  periodicity 
is  far  less  marked  in  seedlings,  with  an  abundant  food- 
supply  in  the  cotyledons  or  in  the  endosperm,  and  in  young 
plants  growing  up  from  bulbs,  tubers,  and  other  parts  in 
which  food  is  stored.  The  growth  of  parasites  and  sapro- 
phytes may  also  vary  periodically  if  they  are  subjected  to 
periodically  varying  conditions.  Light  affects  growth  quan- 
titatively, as  well  as  directing  it  in  the  ways  to  be  described 
in  the  next  chapter  ( p.  208  et  seq. ) .  If  plants  furnished  with 
a  constant  food-supply  are  subjected  to  otherwise  constant 
conditions,  their  growth  rate  will  be  constant  for  a  time. 
For  reasons  not  wholly  understood,  but  certainly  including 
other  factors  than  food-supply,  the  growth-rate  of  any  part 
or  organism  will  rise  to  a  maximum  and  afterward  fall 
again.  For  each  cell  and  for  each  individual  there  is  what 
has  been  rather  pompously  termed  "the  grand  period  of 
growth."  This  means  simply  that  from  its  formation  by 
the  division  of  its  mother-cell  until  the  time  when  it  ceases 
to  increase  in  volume,  each  cell  passes  through  a  period  dur- 
ing which  it  can  grow,  and  during  which  its  rate  of  growth 
gradually  rises  from  nothing  and  falls  again  to  nothing. 
After  growth  ceases,  differentiation  may  still  go  on  as  a 
separate  process.  The  maximum  growth-rate  is  not  neces- 
sarily coincident  with  the  maximum  food-supply  or  with  the 
maximum  of  any  other  tangible  factor. 

Sachs  and  other  physiologists  f  have  called  attention  to 
the  fact,  without  fully  explaining  it,  that  the  growth-rate 

*  Sachs,  J.  von.  Physiology  of  Plants.  Oxford,  1887.  Miss  Gardner 
(Trans,  and  Proceed.,  Bot.  Soc.  Pennsylvania.  Vol.  I,  No.  2.  1901) 
claims  that  the  growth  of  roots  is  faster  by  day  than  by  night.  This  re- 
result  is  probably  due  to  the  favorable  action  of  light  on  processes  upon 
which  growth  depends  rather  than  upon  growth  itself.  The  question  de- 
serves critical  investigation. 

f  Sachs,  J.  von.  Uber  den  Einfluss  der  Lufttemperatur  und  des  Tages- 
lichts  auf  die  stiindlichen  und  tfiglichen  Anderungen  des  Langenwachs- 
thums  (Streckung)  der  Internodien.  Arbeiten  des  bot.  Institute  Wiirzburg, 
Bd.  II.,  1872.  Gesammelte  Abhandlungen ,  Bd.  II.  Lectures  on  the  Physi- 
ology of  Plants,  English  transl.,  p.  552.  Kraus,  Gregor.  Physiologisches 
aus  den  Tropen,  I.  Annales  du  Jardin  Botanique  de  Buitenzorg,  vol.  XII., 
1895. 


GROWTH  171 

does  not  steadily  rise  and  fall  to  and  from  the  maximum, 
but  that  there  are  " discontinuous"  (stossweise)  variations 
apparently  quite  independent  of  the  environment  of  the  or- 
ganism. It  may  be  suggested  that  our  analysis  of  growth, 
according  to  which  it  consists  in  two  distinct  stages — the  one 
fundamental,  in  which  new  protoplasm  is  formed,  the  other 
evident,  in  which  the  cells  expand — may  suggest  a  partial 
explanation.  Without  a  sufficient  number  of  new  cells,  and 
without  a  sufficient  amount  of  new  protoplasm,  no  expan- 
sion can  take  place.  Unless  the  two  processes  keep  pace 
with  each  other,  the  mensurable  one  will  necessarily  be  ir- 
regular. 

In  this  connection  the  fact  already  referred  to  ( pp.1 67, 168 ) , 
that  changes  in  volume  may  take  place  quite  independently 
of  growth  and  because  of  turgor  changes  only,  may  be  con- 
sidered in  somewhat  more  definite  fashion.  Kraus*  pointed 
out  long  ago,  and  has  confirmed  his  observations  made  in 
Europe  by  others  in  the  tropics,  that  there  are  daily  varia- 
tions in  the  length  and  thickness  of  stems  and  branches, 
leaves,  buds,  and  fruits.  "The  diameter  of  a  tree-trunk,  for 
instance,  increases  measurably  till  the  early  morning  hours ; 
it  then  decreases  till  nightfall,  when  it  begins  to  increase 
again."  This  is  due  to  the  variation  in  volume  of  the  cor- 
tical and  other  parenchyma  cells  caused  by  the  difference  in 
the  rate  of  transpiration  at  different  hours  of  the  day.  Ab- 
sorption by  the  root-hairs  continuing  at  a  rate  much  more 
uniform  than  that  of  transpiration  —  at  night  slightly 
higher,  by  day  slightly  lower — the  turgor  and  the  volume 
of  all  living  and  sufficiently  thin-walled  cells  will  vary  ac- 
cordingly. This  variation,  wholly  independent  of  all  vital 
functions,  except  those  which  govern  the  composition  of 
the  cell-sap  and  the  permeability  of  the  protoplasm,  con- 
tinues in  organs  no  longer  growing,  but  may  also,  during 
growth,  contribute  to  the  irregularities  in  the  curve  of 
growth. 

The  otherwise  useful  substances  referred  to  above  (p.  169) 

*  Kraus,  Gregor.  /.  c.  II,  find  earlier  in  Die  Wasservertheilung  in  der 
Pflanze,  1881.  nnd  Die  Gewebespannung  des  Stammes  und  ihre  Folgen. 
Botanische  Zeitung,  1867. 


172  PLANT  PHYSIOLOGY 

are  mainly  water  and  certain  salts  not  directly  entering 
into  the  construction  of  living  protoplasm.  Some  of 
the  water  forms  an  integral  part  of  the  protoplasmic 
structure  ( pp.  6-8 ) ,  but  the  greater  part  of  it  serves  as  the 
vehicle  of  nutritious  substances  brought  to  the  cell,  and  as 
the  solvent  of  all  the  soluble  substances  in  the  cell.  As  the 
essential  and  invariable  ingredient  of  cell-sap,  it  is  the  mate- 
rial which  maintains  the  second  or  evident  stage  of  growth. 
The  volume  of  the  cell  depends  upon  the  water  and  upon 
the  compounds  dissolved  in  it.  The  composition  of  the  cell- 
sap  is  regulated  by  the  living  protoplasm  which  adds  to 
or  takes  from  it  soluble  compounds  of  diverse  sorts, — 
assimilable  and  excreted  matters,  such  as  the  sugars  and 
organic  acids  respectively.  Besides  these,  it  is  claimed  that 
the  cell  owes  its  turgescence  to  certain  soil-constituents, 
especially  the  salts  of  potassium.  According  to  Copeland,  * 
the  degree  of  turgescence  in  ordinary  roots,  stems,  and 
leaves  is  only  slightly  dependent  upon  food-manufacture, 
and  is  mainly  due  to  a  substance  or  to  substances  which 
cannot  be  used  to  keep  the  plant  from  starving.  Copeland 
concludes  from  his  experiments  on  the  effects  of  light  and 
darkness,  heat  and  cold,  that  the  rate  of  growth  has  much 
more  effect  upon  the  turgor  of  the  growing  part  than  vice 
versa. 

The  fundamental  stage  of  growth,  consisting  in  the  for- 
mation of  new  protoplasm,  implies  the  intussusception  or 
interpolation  of  new  particles  between  the  older  parts  of  the 
structure,  or  the  application  or  apposition  of  new  particles 
upon  the  older,  or  both  of  these  processes.  In  either  some 
force  must  be  exerted.  If  the  cell  is  distended  while  the  new 
particles  are  being  formed  and  placed,  the  introduction  of 
new  particles  between  the  older  will  be  proportionally  easier. 
So  the  turgescence  of  the  cell,  tending  to  keep  all  parts 
stretched,  may  contribute  to  such  growth.  But  Copeland 
had  in  mind  evident  growth,  increase  in  volume,  rather 
than  the  formation  of  new  protoplasm.  The  turgor  of 
the  cell  may  be  sufficient  to  stretch  it,  to  increase  its  vol- 

*  Copeland,  E.  B.  fiber  den  Einfluss  von  Licht  und  Temperatur  auf 
den  Turgor.  Inaug.  Diss.,  Halle,  1896. 


GROWTH  173 

ume,  but  this  does  not  necessarily  mean  growth,  as  we 
have  already  seen;  and  again,  if  the  cell  is  stretched,  if 
its  volume  is  increased,  by  other  means,  its  turgor  must 
either  keep  pace  with  this  increase  in  volume,  or  fall.  If  it 
keeps  pace,  because  of  the  composition  of  the  cell-sap  and 
of  the  abundance  of  water  to  be  absorbed,  it  will  be  im- 
possible to  determine  whether  evident  growth  is  dependent 
upon  turgor  and  is  regulated  by  it,  or  not.  If  the  turgor 
fall  during  the  increase  in  volume,  it  must  be  shown  that 
this  fall  is  due  to  no  other  cause.  On  this  point  decisive 
experimental  evidence  is  still  wanting. 

From  experiments  by  True*  on  the  different  rates  of 
elongation  in  the  roots  of  seedlings  grown  in  water-culture 
and  suddenly  transferred  to  culture  media  of  higher  or 
lower  density,  it  would  appear  "that  growth  and  turgor- 
pressure  here  stand  in  no  directly  proportional  relation  to 
each  other."  Furthermore,  Pfefferf  has  shown,  in  a  case 
where  turgor  would  be  at  least  equal!}'  helpful,  namely, 
in  the  formation  of  cell-wall,  that  it  is  not  necessary. 
The  question  resolves  itself  then  into  this :  is  turgor- 
pressure, — so  useful  and  so  necessary  in  maintaining  the 
form  of  cells,  tissues,  organs,  and  organisms— the  force  by 
which  increase  in  volume  is  attained,  or  only  the  means  by 
which  increased  volume  is  maintained?  Apparently  the 
latter  is  more  likely  to  be  the  case ;  but  if  this  is  true,  what 
is  the  force  by  which  the  living  protoplasm  expands,  and 
by  which  it  stretches  its  bounding  walls?  If  turgor  is  not 
the  force  by  which  visible  growth  is  accomplished,  then 
the  increase  in  the  amount  of  water  and  in  the  volume  of 
cell-sap  in  the  growing  part  is  only  the  evidence,  not  the 
intrinsic  quality,  of  visible  growth.  After  all,  we  are  forced 
to  confess  that  the  physiologist's  knowledge  of  the  forces  by 
which  the  living  protoplasm  works  is  very  incomplete. 

Room  is  needed.    Without  it  growth  cannot  take  place. 

*  True.  R.  H.  On  the  influence  of  sudden  changes  of  turgor  and  of 
temperature  on  growth.  Annals  of  Botany,  vol.  9,  1895. 

t  Pfeffer.  W.  Druck  und  Arbeitsleistung  durch  wachsende  Pflanzen. 
Abhandlungen  d.  K.  Sachs.  Gesellsch.  f.  Wissensch.,  Bd.  XX.,  Heft  3.  p. 
429.  1893. 


174  PLANT  PHYSIOLOGY 

Cell-division  may  take  place  even  where  there  is  not  enough 
room  for  growth,  but  it  does  not  constitute  an  essential 
part  of  the  process  of  growth,  though  it  usually  precedes 
growth  and  makes  it  possible.  An  organ  consisting  of  two 
hundred  cells  has  not  grown  in  any  sense  when  these  cells 
by  mere  division  have  become  four  hundred.  Under  normal 
conditions,  however,  growth  will  follow,  increase  in  volume 
and  in  amount  of  protoplasm  taking  place  when  there  is 
enough  room.* 

Growing  parts  exert,  or  may  exert,  great  mechanical 
force.  Illustrations  of  the  truth  of  this  assertion  may  be 
observed  almost  daily.  \  Until  recently,  however,  the  at- 
tempts to  determine  the  amount  of  force  which  a  growing 
part  can  exert  have  yielded  only  inadequate  results.  It  is 
necessary  to  employ  such  apparatus  that  all  the  force  de- 
veloped by  the  growing  plant  will  be  exerted  directly  upon 
the  recording  instrument.  The  best  instrument  so  far  de- 
vised is  Pfeffer's.]; 

Pfeffer's  description  of  his  apparatus  will  explain  the  ac- 
companying illustration,  reduced  from  the  original  figure. 
"The  spring  is  supported  on  an  iron  bar  (d)  14  mm.  thick 
which  is  rigidly  attached  by  means  of  double  screw  clamps 
(e)  to  the  upright  posts  (ss)  of  the  stand.  The  measuring 
spring  (/)  can  be  changed,  for  the  plate  (7)  which  carries  it 
is  fastened  by  the  screws  (kk)  to  the  solid  brass  plate  (#). 
By  raising  or  lowering  this  plate  the  spring  can  be  moved 
up  or  down,  to  or  from  the  plaster-of-Paris  block  (a). 
For  this  purpose  the  plate  (#)  rests  upon  three  screws  (7;) 
which  pass  through  correspondingly  threaded  holes  in  the 
flattened  and  expanded  part  of  the  bar  (d).  Upon  the 
metal  plate  on  the  upper  side  of  the  spring  is  fastened  the 

*  On  the  effects  of  mechanical  restraint  on  the  growth  and  other  behavior 
of  plant-cells  and  parts,  consult  Newcombe,  F.  C.  Influence  of  mechanical 
resistance  on  the  development  and  life-period  of  cells.  Botanical  Gazette, 
vol.  19,  1894.  Pfeffer,  W.  Druck  und  Arbeitsleistung  durch  wachsende 
Pflanzen.  Abhandlungen  d.  K.  Sachs.  Gesellsch.  f.  Wissensch.,  Bd.  XX.,  1893.  i 
Also  p.  187  of  this  book. 

fFor  striking  observations  under  this  topic  see  Kerner  and  Oliver's 
Natural  History  of  Plants.  Vol.  I,  part  2.  pp.  513-17. 

J  Pfeffer,  W.     Druck  und  Arbeitsleistung. 


GROWTH 


175 


glass  plate  ( c )  by  means  of  a  small  amount  of  plaster  of 
Paris.  The  upper  needle  is  fastened  within  the  spring  by 
means  of  shellac,  while  the  lower  one  is  adjustable  by  means 
of  the  screw  (i).  The  flower-pot  (n)  containing  the  root 
set  as  in  the  figure  in  plaster  of  Paris  is  firmly  pressed  into 
the  iron  ring  (777)  fastened  by  two  screws  to  the  upright 
( s ) .  The  small  plaster  block  ( b )  is  now  fitted  to  the  root- 


no.  11. 

Figure  11.    Pfeffer's  apparatus  for  measuring  the  force  exerted  by  grow- 
ing roots  or  stems.     (Reduced  from  the  original  figure.) 

tip  and  fastened  by  plaster  of  Paris  to  the  glass  plate  (c) 
so  that  the  root-tip  is  over  the  middle  of  the  spring.  The 
screws  ( h )  are  now  turned  up  so  that  the  two  plaster  blocks 
(a  and  b)  are  pressed  lightly  together.  The  saw-dust  in 
the  pot  should  be  kept  moist.  The  plaster  blocks  may  be 
constantly  moistened  by  wrapping  them  with  filter-paper 
wet  through  a  strip  of  paper  connecting  with  a  reservoir. 
The  horizontal  rod  (o)  pressed  down  on  the  pot  (n)  and 


176 


PLANT 


fastened  by  screws  to  the  uprights  ( ss )  holds  it  securely  in 
the  ring."  By  appropriate  modifications  of  this  apparatus, 
growth  in  diameter  as  well  as  in  length  can  be  investigated, 
in  stems  as  well  as  in  roots. 

The  following  table  indicates  the  effective  pressures  de- 
veloped by  growing  roots : 

LONGITUDINAL   PRESSURE 


NAME  OF  PLANT. 

PRESSURE  PER  SQ.  MM. 

PRESSURE  IN  ATMOSPHERES. 

Faba  vulgaris 
Zea  mais 
Vicia  sativa 
Aesculus  hippo- 
eastanum 

98+  grams 
138  + 
111  + 

68+ 

9.5 

12.4 
10.7 

6.6 

Mean  =      9.8 

TRANSVERSE    PRESSURE 


Faba  vulgaris 
Zea  mais 

44  + 
68  + 

4.3 

6.5 

These  figures,  obtained  by  averaging  those  in  Tables  I  and 
II  of  Pfeffer's  paper,  indicate  that  although  the  growing 
parts  are  composed  of  such  soft  materials,  they  are  capable 
of  developing  under  resistance  a  force  which  makes  con- 
tinued growth  possible  under  all  ordinary  conditions.  Cross 
and  longitudinal  pressures  developed  by  stems  are  probably 
equal  to  those  developed  by  roots,  but  the  evident  difficul- 
ties in  the  way  of  measurements  as  exact  as  those  for  roots 
cause  the  figures  reported  by  Pfeffer  to  be  somewhat  lower, 
as  for  example,  5.8  and  5.5  atmospheres  for  the  longitu- 
dinal and  cross  pressures  developed  by  the  stems  of  seed- 
lings of  Faba  vulgaris. 

The  growth  of  plants  and  animals  is  as  a  rule  so  slow 
that  accurate  measurements  are  difficult  to  obtain.  Besides 
this,  their  behavior  in  other  ways  is  very  likely  to  compli- 
cate any  attempt  to  determine  either  the  amount  or  the 
rate  of  growth.  The  almost  constant  movement  of  plant- 
parts  out  of  doors  under  the  influences  of  wind,  sunlight, 
warmth,  etc.,  and  the  constant  spontaneous  movements 


GROWTH 


177 


called  circumnutation,  make  ifc  seem  absolutely  necessary  to 
experiment  upon  small  plants  indoors;  but  when  this  is 
done,  other  complications  ensue,  such  as  are  due  to  watering 
the  soil  and  its  subsequent  drying,  the  jarring  of  the  meas- 
uring instruments,  etc.,  etc.  Furthermore,  not  all  plants 
grow  in  straight  lines.  The  tips  of  the  stems  of  many  are 
sooner  or  later  so  curved  that  the  length  of  these  parts  can 
only  be  estimated.  To  overcome  these  various  difficulties 
many  instruments  have  been  devised.  From  the  direct  ob- 
servation and  measurement  of  small  organisms  or  parts  by 
means  of  microscopes,  vertical  or  horizontal,  to  the  com- 
plicated self-recording  auxanometers  of  the  well-equipped 
physiological  laboratory,  the  utmost  variety  in  methods 
and  means  exists.  Illustrated  descriptions  of  instruments 
are  so  accessible  that  space  need  not  be  taken  here  for 
them.*  Most  of  these  instruments  are  but  modifications 
and  improvements  of  those  invented  by  the  masters  in 
plant-physiology,  especially  by  Sachs.  The  principle  of  all 
is  to  magnify  the  growth  so  that  the  evidence  or  the  record 
of  it  is  visible.  The  need  of  this  is  made  evident  by  the 
following  figures — 


Observer. 

Plant. 

Part. 

Growth  per 
minute. 

growth 
per 
minute. 

Truef 

Vicia  Faba 

roots  growing  in 

0.012  mm. 

water 

KrausJ 

Bambusa  sp? 

stalk 

0.040  mm. 

Askenasy  § 

Triticum  sp? 

stamens 

1.05     mm. 

37.5 

Hofmeister^ 

Spirogyra 

cell 

7.5 

Brefeldff 

Coprinus  etercorarius 

stalk 

0.225  mm. 

*  See  Ganong.  in  Botanical  Gazette,  vol.  XXVII.,  1-899. 

Arthur,  XXII.,  1896. 

Stone,  XXII.,  1896. 

Golden,  XIX.,  1894. 

Frost,       '   Minn.  Bot.  Studies,  "        XVII..  1894. 

f  True.  R.  H..  in  Annals  of  Botany,  vol.  IX..  p.  371,  1895.    Figures  in 
Table  I  give  this  average. 

i  Kraus  G..  in  Annales  du  Jardin  Botanique  de  Buitenzorg,  vol.  XII..  1895. 
§  Askenasy.  in  Verhandl.  d.  naturh-  med.  Vereins  in  Heidelberg,   1879. 

*  Hofmeister,  in  Jahreshefte  d.  Vereins  f.  Vaterl.  Naturkunde  in  Wurttem- 
berg,  1874. 

H  Brefeld.  Untersuchungen  iiber  Schimmelpilze.  Heft  3.  1877. 


178 


PLANT  PHYSIOLOGY 


These  numbers,  however,  are  much  above  the  average,  even 
the  Bamboo  being  a  notoriously  rapid  grower.  The  extra- 
ordinarily rapid  growth  of  the  stamens  of  wheat  takes  place 
wrhen  they  are  released  from  mechanical  hindrance  by  the 
spreading  apart  of  the  scales. 

Probably  the  average  rate  of  growth  for  plants  does  not 
exceed,  if  it  equals,  0.005  mm.  per  minute.  Certainly  there 
are  many  plants  which  grow  so  slowly  that  no  one  has  had 
the  patience  and  skill  to  make  accurate  measurements.  The 
lichens  are  among  such  slow  growers,  though  these  must 


GO" 


30,u 


20/i 


70.03 


g  o 


Figure  12.  Curve  of  growth  in  part  of  a  filament  of  Bacillus  ramosus. 
(From  Ward.) 

grow  at  different  rates  as  is  evident  in  the  parts  of  California 
where  the  very  large  " lace-lichen"  (Rainalinti  reticuhita) 
and  crustaceous  and  small  foliose  lichens  live  side  by  side.  * 
In  connection  with  these  figures  as  to  the  rate  and  the 
percentage  of  growth  of  larger  and  in  some  cases  "higher" 
plants,  it  may  be  of  some  interest  to  compare  the  curve  of 
growth  obtained  by  Marshall  Wardf  while  studying  bacte- 
ria. The  accompanying  diagram  gives  the  curve  of  growth 

*  See  Peirce.  G.  J.  On  the  mode  of  dissemination  and  on  the  reticu- 
lations of  Itamalma  reticulata.  Bot.  Gazette,  vol.  XXV.,  1898.  Ditto. 
The  nature  of  the  association  of  alga  and  fungus  in  lichens.  Proc.  -Cal. 
Acad.  Sci..  Series  III.,  Botany,  vol.  I.,  1899.  Ditto.  The  relation  of  fungus 
and  alga  in  lichens.  American  Naturalist,  vol.  XXXIV.,  1900. 

tWard.  H.  M.  On  the  biology  of  Bacillus  ramosus.  Proc.  Roy.  So- 
ciety vol.  LVin..  1895. 


GROWTH  179 

in  part  of  a  filament  of  Bacillus  ramosus,  27.30  /*  *  long  at 
the  beginning,  70.03  :>.  long  at  the  end  of  the  period  of 
observation,  two  hours.  Cell-divisions  occurred  at  the 
points  indicated  by  the  arrows.  Between  the  first  and  the 
second  cell-divisions  there  was  an  increase  of  6.79  /*  in 
length,  between  the  second  and  third  of  9.10  //,  between  the 
third  and  fourth  of  14.56  //.  Between  the  first  and  second 
cell-divisions  there  was  a  lapse  of  thirty-three  minutes,  be- 
tween the  second  and  third,  twenty-six  minutes,  between  the 
third  and  fourth,  thirty-one  minutes,  an  average  of  thirty 
minutes.  The  average  growth  during  these  three  periods 
was  10.14  n  between  each  two  divisions,  or  a  growth  of 
about  one-third  of  a  ,«  per  minute.  This  would  appear  to 
be  slow  growth  in  comparison  with  that  indicated  by  the 
table  on  page  177 ;  but  the  growth  of  many-celled  organ- 
isms represents  the  combined  increase  in  length  accom- 
plished by  a  large  number  of  comparatively  large  cells 
working  together.  Ward's  bacillus  is  a  unicellular  organism 
of  minute  size.  A  moment's  calculation  will  show  that  its 
average  increase  in  length  in  every  thirty  minutes,  that 
is,  between  each  two  cell-divisions,  is  25%.  This  is  certainly 
a  much  higher  rate  of  growth  than  is  possessed  at  any  time 
by  higher  organisms,  t  and  it  is  merely  the  average  rate  dur- 
ing a  half-hour.  Doubtless  its  maximum  growth  is  decided- 
ly higher.  Probably  the  growth-rate  of  bacteria  is  higher 
under  favorable  conditions  than  that  of  any  other  group 
of  organisms.  Their  high  growth-rate,  the  rapidity  with 
which  they  attain  the  size  when  cell-division  is  possible,  the 
promptness  with  which  they  divide,  the  immediate  growth  of 
the  daughter  cells  at  a  high  rate,  all  contribute  to  the 
effectiveness  of  these  minute  organisms. 

It  was  stated  on  page  166  that  each  cell,  each  kind  of 
cell,  and  hence  each  organism,  has  a  maximum  which  its  size 
normally  never  exceeds.  Let  us  seek  a  reason  for  this. 

*  A  fi  or  micron  equals  yoVfr  millimetre. 

+  Since  going  to  press,  a  review  of  Buechner's  paper  (Zuwachsgrossen 
und  Wachsthumsgeschwindigkeiten  bei  Pflanzen.  Dissertation.  Leipzig. 
1901)  has  appeared  (Bot.  Centralbl..  Bd.  90,  p.  500,1902)  in  which  the 
growth  of  the  pollen-tube  of  Imptitiens  Hawkeri  is  reported  at  220%  and 
of  branches  of  higher  plants  as  \%  in  a  unit  of  time  and  length. 


180  PLANT  PHYSIOLOGY 

Each  of  the  component  cells  of  a  multicellular  tissue,  organ, 
or  organism,  is  limited  in  all  its  behavior  by  the  cells  which 
surround  it.  The  single  cell  of  the  unicellular  organism  is 
not  so  limited,  being  constrained  only  by  the  conditions 
prevailing  in  itself  and  in  its  lifeless  surroundings. 

Attempts  have  been  made  to  attribute  the  maximum  size 
ordinarily  attained  by  organisms  to  one  or  two  of  the  three 
conditions  named  on  page  168  as  making  growth  possible. 
It  is  said  that  an  organism  or  a  cell  cannot  grow  beyond 
a  certain  size  because  there  may  not  be  room.  If  this  be 
true,  then  the  word  room  must  be  used  with  a  broader 
meaning  than  that  attached  to  it  in  our  discussion  on 
pages  173  and  174 :  it  must  mean,  as  stated  on  page  6, 
freedom  from  interference  of  every  sort.  This  last  is  un- 
doubtedly true,  for  a  plant  with  such  an  impulse  to  grow 
that  it  might  otherwise  cover  the  whole  earth  would  be 
prevented  by  the  presence,  if  not  by  the  attacks,  of  the  other 
organisms  living  at  the  same  time.  So  the  individual  is 
kept  within  a  certain  size. 

Again,  it  is  said  that  nutrition  fixes  the  limit  of  growth. 
In  a  growing  spherical  cell,  the  increase  in  surface  and  in 
mass  are  to  each  other  as  the  square  to  the  cube,  "in  other 
words,  the  smaller  the  cell,  the  greater  is  the  surface  in 
proportion  to  the  mass;  and  the  more  the  cell  grows,  the 
less  does  the  surface  grow  in  proportion  to  the  mass."*  It 
is  said  that  since  all  food-materials  are  taken  in  through 
the  surface,  the  supply  of  food  will  be  insufficient  when  the 
surface  becomes  too  small  in  proportion  to  the  mass.  This 
implies,  however,  that  the  absorbing  power  does  not  in- 
crease proportionally  with  the  mass.  The  absorbing  power, 
as  we  have  seen,  is  the  osmotic  force  exercised  by  the  cell- 
sap  upon  the  solutions  and  the  constituents  of  the  solu- 
tions outside  the  cell.  This  osmotic  force  depends  upon  the 
differences  in  the  composition,  absolute  and  proportional,  of 
-oil-sap  and  surrounding  liquid.  The  larger  the  volume  of 
cell-sap,  the  more  slowly  can  it  be  made  like  the  liquid  out- 
side, i.  e.  the  more  slowly  will  the  osmotic  force  be  dimin- 

*  Verworn,  M.  Allgemeine  Physiologie,  Ite  Aufl.,  p.  511,  1895.  Engl. 
transl.  by  Lee,  General  Physiology,  p.  530.  1899. 


GROWTH  181 

ished.  We  have  no  reason  to  think  that  the  osmotic  force 
is  less,  for  the  turgor  is  not  lower  in  large  than  in  small 
cells.  So  it  cannot  be  merely  the  ratio  of  surface  to  mass 
which  is  the  determining  factor. 

The  high  turgor  and  osmotic  force  maintained  in  large 
cells  imply,  besides  the  density  of  the  cell-sap,  that  the 
enclosing  membranes  of  the  cells  are  not  freely  permeable. 
Upon  the  impermeability  of  the  membranes  and  upon  the 
composition  of  the  cell-sap  depend  the  turgescence,  the 
plumpness,  of  cells  of  large  size.  The  membranes  must 
possess  strength  as  well  as  impermeability,  but  it  is  their 
impermeability  which  makes  it  necessary  that  they  should 
also  be  strong.  It  is  this  change  in  the  quality  of  the  sur- 
face rather  than  in  the  mere  ratio  of  surface  and  mass, 
which  is  the  important  factor  in  limiting  size.  But  this  is 
not  all. 

Whatever  may  be  the  distinct  functions  of  nucleus  and 
cytoplasm,  it  is  safe  to  conclude  that  neither  can  be  greatly 
increased  or  diminished  in  amount  or  in  activity  without 
affecting  the  other  and  the  cell  as  a  whole.  By  chilling  cul- 
tures of  Spirogyra  while  in  a  state  of  cell-division,  Geras- 
simow  *  regularly  secured  the  formation  of  cells  without 
nuclei,  normal  cells,  and  cells  with  double  the  usual  amount 
of  nuclear  substance.  He  found  that  the  non-nucleated  cells 
grew  very  slightly  in  length,  that  normal  cells  grew  nor- 
mally, that  cells  with  more  than  the  normal  amount  of  nu- 
clear substance  attained  a  larger  size  and  divided  later  than 
normal  cells.  Since  the  volume  of  the  nucleus  does  not  keep 
pace  with  the  volume  of  the  cytoplasm  or  of  the  cell  as  they 
both  increase  shortly  after  the  cell  is  formed  by  division, 
the  disparity  in  the  amounts  of  nuclear  and  cytoplasmic 
substances  increases.  It  is  conceivable  that  growth  ceases 
when  the  amount  of  cytoplasmic  in  proportion  to  nuclear 
substance  has  attained  the  optimum  or  maximum ;  in  other 
words,  that  the  limit  of  growth  is  fixed  in  the  first  and  fun- 
damental stage,  the  subsequent  increase  in  size  going  only 
to  the  limit  set  by  the  amount  of  protoplasm  formed. 

*  Gerassimow.  J.  J.  Uber  den  Einfluss  dee  Kerns  auf  das  Wachsthum 
der  Zelle.  Moskau,  1901. 


182  PLANT  PHYSIOLOGY 

If  the  nucleus  were  larger  and  the  cytoplasm  proportion- 
ally abundant,  if  the  permeability  of  the  enveloping  mem- 
branes and  their  tensile  strength  were  proportionally  in- 
creased, if  the  absorbent  power  (osmotic  force)  of  the  cell 
were  also  raised  proportionally,  is  there  any  reason  why  the 
cell,  the  organ,  and  the  organism  should  not  grow  larger? 
The  question  cannot  be  answered.  Many  organisms  do  not 
attain  larger  size  when,  so  far  as  we  can  now  see,  it  would 
•be  possible  for  them  to  do  so.  Are  the  bacteria  so  small  be- 
cause they  have  so  little  nuclear  substance  in  proportion  to 
cytoplasmic?  On  the  contrary  some  authors*  have  claimed, 
from  the  behavior  of  bacterial  cells  toward  staining  agents, 
that  they  are  mainly  nuclear  substance  writh  but  a  thin  layer 
of  enveloping  cytoplasm.  The  amount  of  room  and  of  food 
which  the  individual  bacteria  could  occupy  and  consume 
would  certainly  suggest  that  they  might  attain  larger  size. 
Their  enveloping  membranes  appear  to  be  sufficiently  per- 
meable and  strong  for  larger  organisms.  Yet  the  bacteria 
remain  minute,  and  no  reasons  now  known  can  account  for 
their  size.  If  our  mechanical  explanations  fail  on  these  organ- 
isms, are  they  any  more  certain  to  be  correct  when  applied 
to  the  growth  of  larger  ones?  What  limits  the  size  must 
have  to  do  with  the  sensitiveness,  the  irritability,  of  the 
living  matter,  and  this  leads  us  to  the  subject  of  the  next 
chapter. 

*  See  in  Migula's  System  der  Bakterien,  Bd.  I.,  pp.  72-80.  the  discussion 
of  this  point  and  the  references  pro  and  con. 


CHAPTER    VI 
IRRITABILITY 

The  preceding  chapters  have  taught  us  that  living  or- 
ganisms are  composed  of  chemical  compounds  and  that  they 
work  by  physical  force.  The  body  of  a  living  plant  consists 
of  living  protoplasm  and  of  lifeless  substances  The  func- 
tions of  a  living  plant  consist  in  chemical  changes  some  of 
which  liberate  energy,  or  store  it,  while  others  result  in  the 
accumulation  of  matter,  lifeless  and  living.  These  functions 
are  carried  on  by  the  living  organism,  they  do  not  simply 
take  place ;  but  the  organism  lives  and  carries  on  these  func- 
tions only  by  using  chemical  compounds  and  physical  forces. 
Just  as  chemical  and  physical  processes  are  affected  by  pre- 
vailing conditions,  so  the  living  organism  is  affected  by  each 
factor  of  its  environment.  When  the  factors  change,  the 
organism  is  differently  affected,  just  as,  with  changing^ 
conditions,  ordinary  chemical  and  physical  processes  also 
change  correspondingly.  As  there  is  an  optimum  condition, 
which  consists  hi  temperature,  illumination,  supply  of  water 
and  of  other  substances,  etc.,  for  each  chemical  reaction 
taking  place  in  the  laboratory  and  in  nature,  so  there  is  an 
optimum  condition  for  that  complex  of  chemical  reactions 
constantly  taking  place  hi  the  actively  living  organism.  - 
Any  departure  from  the  optimum  modifies  some  or  all  of  the 
chemical  changes  in  the  organism  so  that  there  is  a  different 
and  less  favorable  balance  in  the  complex.  Conversely,  any 
approach  to  the  optimum  condition  so  modifies  some  or  all 
of  the  chemical  changes  that  their  balance  is  more  favor- 
able. When  we  conceive  a  living  organism,  even  the  simplest 
and  smallest,  as  being  a  definite  structure  (protoplasm) 
consisting  of  simple  water  molecules  and  of  other  molecules 
highly  complex  and  therefore  comparatively  destructible, 


184  PLANT  PHYSIOLOGY 

built  together  and  enclosing  many  other  compounds,  simple 
or  complex,  we  have  the  ground- work  for  a  rational  concep- 
tion of  the  sensitiveness  of  living  organisms  to  their  sur- 
roundings, that  is,  of  their  irritability. 

A  solid  mass  of  the  metals  and  jewels  ordinarily  employed 
in  the  construction  of  a  chronometer  may  be  subjected  to 
much  harsher  treatment  without  danger  of  destruction  than 
the  same  weight  of  the  same  substances  arranged  as  a 
chronometer.  Though  the  substances  are  the  same,  their 
arrangement  in  the  two  cases  is  the  reason  for  their  sensi- 
tiveness, or  their  power  of  resisting  violence.  Living  sub- 
stance, protoplasm,  is  a  structure  infinitely  finer  and  hence 
more  delicate  than  a  chronometer.  Furthermore,  proto- 
plasm is  composed  of  many  more  chemical  compounds  than 
those  entering  into  the  structure  of  a  chronometer.  The 
molecules  of  each  of  these  compounds  are  composed  of  so 
many  atoms  of  so  many  different  elements  that  they  are  far 
less  coherent  and  stable  than  the  simple  one,  or  two,  or 
three  atomed  molecules  forming  the  substances  in  a  chro- 
nometer. Besides  all  this,  these  large  numbers  of  atoms  are 
combined  into  molecules,  these  complex  molecules  are  ar- 
ranged in  groups,  these  groups  enclose  and  are  surrounded 
by  water,  and  the  water  holds  in  solution  oxygen  and 
a  variety  of  compounds.  The  component  atoms  of  these 
compounds  have  affinities  for  other  atoms  as  well  as  for 
those  with  which  they  are  combined.  Furthermore,  the  sub- 
stances may  not  all  be  in  the  molecular  state  in  the  solu- 
tion; the  component  atoms  of  some  substances  may  be 
more  or  less  completely  dissociated.  *  In  this  condition  they 
will  be  still  more  susceptible  to  physical  and  chemical  influ- 
ences than  if  combined  into  molecules,  and  they  will  affect 
the  protoplasm  with  correspondingly  greater  promptness. 
The  intimate  contact  of  the  aqueous  solution,  the  cell-sap, 
with  the  living  protoplasm,  and  the  complete  distribution  of 

*See  Ostwald,  W.  Outlines  of  general  chemistry,  Eng.  transl.  by 
Walker,  London  and  New  York,  1895.  Nernst,  W.  Theoretical  chem- 
istry, Eng.  transl.  by  Palmer,  London  and  New  York,  1895.  Jones. 
H.  C.  The  theory  of  electrolytic  dissociation  and  some  of  its  appli- 
cations. New  York,  1900. 


IRRITABILITY  185 

the  solution  throughout  the  cell,  insure  the  thoroughness 
with  which  the  protoplasm  will  be  affected.  The  complexity 
of  protoplasm  hi  composition  and  structure,  and  the  physi- 
cal and  chemical  properties  of  the  cell-sap  which  is  every- 
where within  it,  help  us  to  see  that  protoplasm  is  neces- 
sarily as  well  as  actually  the  most  unstable  and  the  most 
complex,  structure  known. 

Comprehending  these  facts,  we  see  reasons  for  the  sensi- 
tiveness of  protoplasm  to  outside  influences.  But  lifeless 
protoplasm — imagining  such  a  thing  for  the  moment — al- 
though it  possesses  all  this  complexity  of  structure  and  of 
composition,  and  is  therefore  sensitive  to  influences  from 
without,  is  not  the  seat  of  the  physiological  processes,  of 
the  destructive  and  constructive  chemical  changes,  wrhich  are 
constantly  going  on  in  living  protoplasm.  Indeed  the  dry 
seed  is  far  less  sensitive,  as  we  have  already  seen  (pp.  9,  10 ), 
than  the  same  seed  after  water  has  been  absorbed  and  ger- 
mination has  begun.  Wherever  there  is  actively  living  proto- 
plasm, i.e.  a  complex  structure  among  the  compounds  of 
which  and  within  which  chemical  changes  are  constantly 
taking  place,  we  have  the  conditions  for  irritability :  the 
more  complex  the  structure  and  its  component  and  enclosed 
compounds,  or  the  more  varied  and  the  more  rapid  the 
chemical  changes  taking  place  hi  the  structure,  the  greater 
will  be  its  sensitiveness  to  external  influences.  The  higher 
the  organism,  the  more  complex  is  its  structure,  the  more 
varied  or  the  more  rapid  are  the  chemical  changes  taking 
place  in  it,  and  therefore  it  is  the  more  sensitive.  The  or- 
ganism is  dormant,  unsensitive,  unirritable,  when  its  physi- 
ological processes  are  slow  or  simple ;  the  organism  is  dead 
when  its  physiological  chemical  changes  cease  and  its  struc- 
ture breaks  down.  Its  component  molecules  may  still  be 
there  intact  after  the  organism  ceases  to  live,  but  their  ar- 
rangement is  changed,  the  sensitiveness  of  the  whole  struc- 
ture and  of  all  its  parts  is  diminished  in  proportion  as  the 
arrangement  of  the  molecules  is  modified. 

The  irritability,  then,  of  living  organisms  consists  in  a 
sensitiveness  to  external  conditions  which  is  due  to  the 
complexity  in  structure  and  in  composition  of  the  organ- 


X^13T*AU5S, 
f  Of  THE 

G    UNIVERSITY   J 

\  OF  J 


186  PLANT  PHYSIOLOGY 

isms  themselves.  To  put  this  in  more  definite  terms  we  may 
say  that  the  irritability  (which  we  may  represent  as  x)  of 
a  cell,  an  organ,  or  an  organism,  consists  in  the  sum  of 
these  factors,  viz. — 

(a)  the  sensitiveness  of  the  component  atoms  of  complex 
molecules  to  other  forces  and  affinities  as  well  as  to  the 
affinities  which  hold  them  together  in  these  compounds. 

(b)  the  instability  of  the  groups  of  molecules. 

(c)  the  instability  of  the  protoplasmic    structure  which 
consists  of  water  and  these  complex  molecules. 

(d)  the   number,    variety,    and    speed    of    the    chemical 
changes  taking  place  in  this  structure. 

(e)  the   number,    variety,    and    speed    of    the    chemical 
changes  taking  place  between  its  component  molecules  and 
others  enclosed  among  them  or  outside. 

( f)  the  number,  kinds,  and  degree  of  dissociation,  of  the 
atoms   and    molecules    of  the  substances  dissolved  in  the 
water  in  the  cell. 

Thus  x='a  +  5  +  c+d+e  +  /*,  a  sum  greater  than  is 
attained  in  any  known  combination  except  the  living  cell. 

Let  us  pass  on  now  from  these  general  considerations  as 
a  starting-point,  to  examine  certain  phases  of  irritability : 
first,  the  relations  of  irritability  to  the  amount,  kind,  etc., 
of  growth ;  second,  the  direction  of  growth,  and  movement, 
as  depending  upon  irritability;  third,  the  growth  move- 
ments not  evidently  connected  with  irritability. 

IRRITABILITY  AND  THE  AMOUNT  AND  KIND  OF  GROWTH 

Every  actively  growing  organism  must  have,  besides  an 
adequate  supply  of  material,  an  adequate  amount  of  room, 
and  the  impulse  to  grow  ( see  p.  168 ) ,  at  least  the  power  to 
direct  its  growth  according  to  its  environment.  This  power 
is  dependant  upon  the  irritability  of  the  organism  and  of  its 
separate  organs,  their  sensitiveness  to  forces  and  influences 
wholly  external.  Since  the  material  of  which  it  is  composed 
came  into  existence,  every  organism  has  been  subjected  to 
external  influences,  some  momentary  or  unequal,  some  per- 
sistent and  uniform.  The  effects  of  these  influences  are  more 
or  less  enduring,  like  the  influences  themselves,  but  presuma- 


IRRITABILITY  187 

bly  are  far  from  permanent  at  the  utmost.  While  the  effect 
of  one  influence  still  persists,  other  influences  are  operating 
on  the  organism.  The  status  of  the  organism  at  any  one 
time  represents  the  results  of  all  the  influences  to  which  it 
has  been  subjected  up  to  that  time;  its  form,  size,  position, 
etc.,  are  its  response  to  all  these  influences.  The  condition 
of  the  organism  and  the  influences  bearing  upon  it,  together 
determine  how  it  will  grow.  The  study  of  irritability  at 
any  one  time,  therefore,  necessarily  includes  the  results  of 
irritability  at  other  times  earlier  in  the  life  of  the  in- 
dividual. 

We  may  begin  our  study  with  those  mechanical  influences 
to  which  plants  are  or  may  be  subjected.  When  a  growing 
part  is  enclosed  within  a  bandage  of  plaster  of  Paris,  two 
results  follow,  one  of  which  has  already  been  mentioned 
(p.  174).*  The  other  wre  may  consider  now.  The  part  not 
only  lacks  room  to  grow,  but  the  plaster  ligature  relieves 
the  enclosed  part,  as  it  does  a  broken  arm  or  leg,  of  me- 
chanical strain  of  nearly  every  sort.  The  tissues  primarily 
contributing  to  the  mechanical  strength  of  the  growing 
organ  attain  within  the  ligature  neither  such  size  nor  such 
strength  as  ordinarily,  f  From  this  we  may  conclude  that 
the  mechanical  strength  of  a  part  depends  upon  the  strain 
to  which  it  is  subjected.  This  conclusion,  reached  from  ex- 
periments in  which  the  mechanical  strain  was  reduced  as 
much  as  possible,  is  enforced  by  experiments  of  the  opposite 
sort,  in  which  the  mechanical  strain  was  increased.*  The 
strain  consisted  in  traction,  effected  by  means  of  weights 

*Newcomb.  F.  C.  The  influence  of  mechanical  resistance  on  the  de- 
velopment and  life-period  of  cells.  Botan.  Gazette,  vol.  19,  1894.  Reg- 
ulatory formation  of  mechanical  tissue.  Ibid.  vol.  20.  1895. 

f  Pfeffer.  W.  Druck  und  Arbeitsleistung.  Abh.  d.  K.  Sachs.  Gesellsch.  f. 
Wissensch.,  Bd.  XX.,  1893.  Also  Pflanzenphysiologie.  2te  Aufl..  II.,  pp. 
144-7.  1901. 

%  Hegler,  R.  Einfluss  des  mechanischen  Zugs  auf  das  Wachsthum  der 
Pflanze.  Cohr's  Beitrage  zur  Biologic  der  Pflanzen.  Bd.  VI..  1893.  Older 
literature  here  cited.  See  also  Pfeffer.  W.  Besprechung  Hegler's  Unter- 
suchungen.  Berichte  d.  K.  Sachs.  Gesellsch.  f.  Wissensch..  Sitzung  vom  7ten 
Dec.,  1891.  A ko Pflanzenphysiologie  II..  §36.  Derschau.  M.  von.  Einfluss 
von  Kontakt  und  Zug  auf  rankende  Blattstiele.  Inaug.-Diss..  Leipzig.  1893. 


188 


PLANT  PHYSIOLOGY 


suspended  from  threads  passing  over  pulleys  and  fastened  to 
growing  parts.  The  following  figures  will  illustrate  the 
results  obtained — 


WEIGHT. 

SEEDLINGS. 

PETIOLES. 

Helianthus. 

Phaseolus. 

Heleborus. 

Enough  to  break. 

160  gr. 

180  gr. 

400  gr. 

Used  to  strengthen 

150    " 

165    " 

"    test  on  2d  day. 

250    " 

"      "      «   3d     - 

300    " 

"      "     after  several  days, 

400    " 

"      "     on  7th  day, 

650    " 

«      «      «    5th     ci 

3^  K. 

Subjecting  otherwise  weak  stems  to  pull  induces  them  to 
form  strengthening  tissues  which  would  not  ordinarily  de- 
velop at  all.  Stems  thus  acted  upon  decrease  their  growth 
in  length  in  proportion  as  they  are  stimulated  to  grow  in 
thickness.  Even  a  strain  too  slight  to  produce  any  stretch- 
ing will  have  this  effect. 

The  formation  of  strengthening  tissues  is  proportional  to 
the  need.  This  is  shown  each  year  by  fruiting  plants.  The 
fertilization  of  the  egg-cells  in  the  ovules  of  flowering  plants 
leads  to  the  production  of  seeds,  the  growth  of  fruit,  and 
the  considerable  increase  in  weight  of  these  and  of  the  adja- 
cent parts.  The  development  of  the  fruit  and  its  contents 
demands  a  corresponding  growth  both  of  conducting  and 
also  of  mechanically  strengthening  tissues  extending  into 
regions  quite  distant  from  the  fruit  as  well  as  in  those  near 
it.*  A  similar  response  to  mechanical  strain  is  shown  by 
many  mosses  and  liverworts.  The  stalks  bearing  the  fruits 
grow  greatly  in  length  for  a  time.  Later,  when  the  fruits 
increase  in  weight,  the  stalks  cease  to  elongate  and  become 
much  thicker  and  stronger. 

Plants  are  everywhere  in  nature  exposed  to  mechanical 
strains  of  more  or  less  force  and  constancy.  The  winds, 
flowing  water,  tides,  waves,  and  the  movements  of  animals 

*Pieters.  A.  J.  Influence  of  fruit-bearing  on  the  development  of  me- 
chanical tissue  in  some  fruit  trees.  Annals  of  Botany.  X.  1896. 


IRRITABILITY  189 

apply  mechanical  force  to  living  plants,  and  to  these  influ- 
ences they  respond  by  the  increased  or  modified  activity  of 
the  living  protoplasm. 

The  most  evident  effects  of  winds  are  those  deformities 
which  usually,  however,  are  more  the  result  of  injury  than 
of  stimulus.  Trees  in  exposed  places  are  unsymmetrical, 
their  limbs  short  and  broken  on  the  side  toward  the  strong 
or  violent  prevailing  wind,  while  on  the  other  side  the  limbs 
look  as  if  they  had  been  drawn  along  with  the  wind.  But 
the  root-syste  n  of  such  trees  shows  the  stimulating 'effect  of 
the  wind  without  the  deformities  exhibited  in  the  branches, 
the  roots  being  longer  and  stronger  on  the  windward 
than  on  the  leeward  side,  the  greatest  strength  develop- 
ing where  there  is  the  greatest  mechanical  force  to  be 
resisted. 

The  ordinary  swaying  of  stems  and  branches,  and  even  of 
leaves  on  their  stalks,  acts  as  a  stimulus  to  the  living 
cells  of  a  plant.  The  difference  in  the  amounts  of  mechan- 
ical tissue  in  plants  which  carry  their  own  weight  and  in 
those  which  lean,  twine,  and  otherwise  climb,  is  partly  due 
to  the  difference  in  mechanical  stimulus  to  form  strength- 
ening tissues.  By  tying  an  erect  plant  so  firmly  to  a  sup- 
port that  it  cannot  sway  in  the  wind,  or  by  supporting  its 
weight  on  a  frame,  the  plant  will  be  deprived  of  those  move- 
ments, stresses,  and  strains  which  normally  stimulate  it 
to  develop  strength. 

Water-currents  exercise  similar  effects  to  those  of  wind- 
currents,  whenever  they  are  rapid  enough  to  develop  any 
considerable  force.  Even  slow  water-currents  have  been 
found  to  stimulate  and  direct  growth  and  movement  in 
rather  peculiar  fashion.  Thus  the  roots  of  certain  plants,  if 
suspended  in  clean  running  water,  will  bend  so  that  the  tips 
point  and  grow  up  stream.  This  phenomenon  is  known  as 
rheotropism.  *  The  plasmodia  of  certain  Myxomycetes  will 
grow  on  a  vertical  strip  of  filter-paper  always  in  the  direc- 
tion opposite  to  that  of  the  current  of  water  which  is  sup- 

*  Juel,  H.  O.  Untersuchungen  iiber  den  Rheotropismus  der  Wurzeln. 
Jahrb.  f.  wiss.  Bot.,  Bd.  34.  1900.  Newcombe.  F.  C.  Rheotropism  cf 
roots.  Bot.  Gazette  vol.  33  1902. 


190 


PLANT  PHYSIOLOGY 


plied  to  them,  whether  this  be  upward  or  downward.  This 
phenomenon  is  called  rheotaxis,  *  but  it  is  not  certain  that 
it  is  not  a  response  to  obscure  chemical  stimuli  rather 
than  to  a  current  of  water  merely.  Rheotaxis  and  rheo- 
tropism  are  therefore  probably  distinct  phenomena.  The 
significance  of  rheotropism  is  not  understood. 


FKJ.  13. 

Figure  13.  PosteMa  Palmspformis.  Sea-palms  at  Point  Lobos,  near 
Monterey,  California.  Height  about  2  feet.  Photograph  by  Dr.  W.  A. 
Shaw. 

Waves  and  tides  have  not  been  studied  experimentally  in 
their  mechanical  relation  to  plants.  It  may  be  inferred, 
perhaps,  that  they  produce  movements  which  stimu- 
late the  plants  exposed.  Certainly  plants  which  are  to 
withstand  the  pounding  of  the  waves  must  grow  propor- 
tionally resistant.  Is  this  simply  a  case  of  the  survival 
of  the  accidentally  toughest  and  fittest,  or  do  tide  and  surf 
plants  irritably  react  to  the  rude  stimuli  to  which  they  are 


Stahl,  E.     Zur  Biologic  der  Myxomyceten.    Bot.  Zeitung,  1884. 


IRRITABILITY  191 

subjected?  Of  these  buffet-ted  forms  the  Sea  Palms  (Pos- 
telsia  paJm&formis)  of  the  Pacific  Coast  are  the  most  strik- 
ing. Living  between  the  tide-marks,  always  in  the  most 
exposed  positions,  these  upright  plants  hold  on  and  grow  in 
spite  of  the  tremendous  pounding  to  which  they  are  almost 
continually  exposed.  In  toughness,  strength,  and  elasticity 
their  upper  parts  are  equalled  only  by  the  closeness  of  the 
attachment  and  the  strength  of  the  hold-fasts.  The  ,accom- 
panying  figure  suggests  how  rough  their  habitat  may  be  in 
a  storm. 

From  the  foregoing  we  may  conclude  that  a  certain 
amount  both  of  freedom  to  move  and  also  of  actual  agita- 
tion is  good  for  plants.  This  is  exercise,  apparently  as  de- 
sirable for  plants  as  for  animals,  and  presumably  for  the 
same  reasons.  It  facilitates  the  transfer  of  nutrient  sub- 
stances and  it  stimulates  the  living  protoplasm.  Which 
factor  is  the  more  important  it  remains  for  experiment  to 
determine. 

In  trees  and  shrubs  the  mechanical  tissues  are  found  espe- 
cially in  the  wood.  Where  the  seasons  are  sharply  con- 
trasted, as  over  the  greater  part  of  the  temperate  zones,  the 
wood  presents  the  familiar  appearance  known  as  annual 
rings.  In  mechanical  strength  the  different  parts  of  the 
wood  vary  considerably,  the  so-called  ''spring- wood,"  be- 
cause of  the  larger  size  of  the  cells  and  the  comparative 
thinness  of  their  walls,  being  decidedly  weaker  than  the 
thicker-walled,  more  compact,  and  often  more  abundant 
''autumn  wood."  It  is  through  the  wood,  especially  the 
ducts  and  tracheids,  that  the  transfer  of  food-materials 
from  roots  to  leaves  takes  place  (see  pp.  119-124).  The 
wood  is,  therefore,  both  a  mechanical  and  a  vascular  tissue. 
The  one  function  or  the  other  predominates  at  different 
times  during  the  growing  season  and  affects  the  growing 
and  developing  tissues  accordingly. 

Where  growth  is  always  possible  and  is  practically  con- 
tinuous, annual  rings  are  not  formed.  It  is  only  where 
growth  is  periodic  because  of  changing  seasons,  like  winter 
and  summer,  dry  and  rainy  seasons,  that  there  are  decided 
differences  in  the  character  of  the  wood.  Certain  other 


192  PLANT  PHYSIOLOGY 

phenomena  almost  or  quite  coincide  with  the  formation  of 
annual  rings.  "Spring  wood"  forms  (seep.  123)  when  sap- 
pressure  is  greatest,  when  the  buds  open  and  the  leaves 
expand,  when  there  is  a  sudden  extension  of  the  surface 
from  which  water  will  evaporate.  At  this  time  water  must 
be  abundantly  supplied  to  the  parts  just  emerged  from  the 
bud  so  that  the  new  cells  may  expand  to  their  proper  size ; 
food  must  be  furnished  these  growing  parts  so  that  new 
cells  and  new  protoplasm  may  form  and  the  parts  may  con- 
tinue to  increase  in  size.  When  the  buds  unfold  there  is  an 
immediate  and  great  demand  upon  the  conducting  tissues, 
but  as  the  parts  increase  in  size  and  weight,  the  mechanical 
strength  of  branchlets,  branches,  and  stem  must  increase 
also.  With  an  increasing  weight  each  spring  and  early  sum- 
mer there  is  an  annually  increasing  mechanical  strain  upon 
the  tree  or  shrub.  This  increased  strain  is  yearly  met  by 
increased  strength,  and  this  is  contributed  largely  by  the 
"autumn  wood." 

The  time  during  which  the  cambium  cells  give  rise  by 
division  to  new  cells  differentiating  into  wood  and  bast  ele- 
ments is  much  briefer  than  the  season  during  which  growth 
is  apparently  possible.  According  to  Jost,  *  the  greater  part 
of  the  increase  in  thickness  of  stems  and  branches  takes 
place  in  May  and  June  ( in  Germany ) .  This  indicates  that 
the  activity  of  the  cambium  cells  and  of  their  immediate 
derivatives  is  controlled  by  influences  outside  of  themselves. 
These  influences  are  doubtless  many,  but  we  may  distinguish 
some  of  them  at  least. 

The  young  parts  growing  and  developing  from  opening 
buds  in  the  spring  need  much  food  and  water,  and  they 
certainly  transpire  greater  or  less  quantities  of  water-vapor. 
There  is  at  this  time  an  especially  great  and  a  fairly  steady 
demand  upon  the  conducting  tissues  for  both  food  and 
water,  not  so  much  for  transpiration,  perhaps,  as  for 
growth  in  the  full  sense  of  the  word;  for  food  so  that  new 
protoplasm  may  be  formed,  for  water  so  that  it  may  prop- 
erly expand.  This  demand  would  make  itself  felt  first  in  the 

*  Jost,  L.  Beobachtungen  iiber  den  zeitlichen  Verlauf  des  Dickenwachs- 
thums  der  Baume.  Ber.  d.  Deutsch.  Bot.  Gesellsch.,  Bd.  X.,  1893. 


IRRITABILITY  193 

parts  nearest  opening  buds.  It  is  here  that  the  cambium 
first  resumes  its  activity,  the  more  and  more  distant  parts 
coming  only  successively  into  activity  again.*  It  is  pre- 
cisely the  parts  nearest  the  fully  expanded  leaves  and  the 
maturing  terminal  buds  in  which  the  cambium  also  first 
ceases  to  be  active.  In  plants  which  form  no  terminal  buds 
— such  as  roses,  briars,  etc. — the  cambium  continues  to  be 
active  so  long  as  the  temperature,  moisture,  and  other  ex- 
ternal conditions  make  growth  possible.  The  cambium  cells 
divide  more  or  less  early,  and  a  larger  or  smaller  number  of 
times.  To  a  considerable  extent  at  least  this  is  according 
to  the  behavior  of  the  parts  developing  from  the  opening 
buds.  The  living  cells  in  rapidly  growing  leaves  and  elon- 
gating internodes,  demanding  much  food  and  water,  stim- 
ulate by  this  demand  the  earliest  cells  cut  off  by  division 
of  the  cambium  cells  to  grow  to  such  size  and  to  take  on 
such  characters  that  they  will  best  conduct  what  is  needed 
above.  If  the  ground  is  so  dry  that  only  insufficient  quanti- 
ties of  water  can  be  absorbed,  or  if  in  the  preceding  year 
only  insufficient  quantities  of  food  were  made  and  stored, 
the  parts  coming  from  the  opening  buds  will  develop  less 
rapidly  or  less  perfectly  in  size,  etc.,  and  will  be  able  to  exert 
and  will  exert  less  of  a  stimulating  demand  upon  the  con- 
ducting tissues  for  food  and  water  than  in  a  better  season. 
In  this  way  nutrition  affects  everything,  the  formation  of 
wood  as  well  as  the  development  of  new  organs.  In  the 
various  living  cells  composing  the  embryonic  organs  in  the 
bud,  the  impulse  to  grow  is  given  by  returning  favorable 
conditions.  Warmth,  light,  moisture,  etc.,  stimulate  the 
cells  to  grow,  to  divide,  to  grow  again,  to  differentiate, 
etc.  These  cells,  stimulated  by  purely  physical  influences 
from  outside  themselves,  develop  needs  proportioned  to 
their  physiological  activities.  The  needs  must  be  met,  if  the 
cells  are  to  continue  their  activities,  by  materials  drawn 
from  their  neighbors.  So  this  demand,  extending  from  cell 
to  cell  by  the  osmotic  transfer  of  nutrient  solutions,  pres- 
ently reaches  the  living  cells  adjoining?  the  conducting  ele- 

*  Jost.  L.   Beziehungen  zwischen  der  Blattentwickelung  und  der  Gefass- 
bildung  in  der  Pflanze.    Bot.  Zeitung,  1893. 
13 


194  PLANT  PHYSIOLOGY 

ments,  the  cambium  and  its  daughter  cells.  According  to 
the  demand,  the  stimulus,  thus  exerted,  these  daughter  cells 
develop  into  the  so-called  "spring  wood." 

The  hypothesis  thus  outlined  would  be  worthless  if  it  were 
not  for  the  early  growth  of  the  roots  which  makes  possible 
the  supply  of  the  relatively  large  volume  of  water  demanded 
by  the  parts  coming  from  the  bud.  Goff*  has  recently 
shown  that  growth  of  the  root  begins  in  the  spring  before 
there  are  any  signs  of  growth  in  the  parts  above  ground. 
Thus  the  plant  is  early  provided  with  the  absorbing  agent 
needed.  This  growth  of  the  root  also  must  be  regarded  as 
the  irritable  response  to  the  stimulus  exerted  upon  it  by  the 
moisture  and  the  increasing  warmth  of  the  soil. 

The  other  half  of  the  annual  ring  remains  to  be  accounted 
for.  What  has  already  been  said  regarding  the  effect  of  in- 
creasing the  mechanical  strains  to  which  growing  parts  are 
subjected  (see  pp.  174,  187-8)  prepares  us  for  an  hypothe- 
sis, deserving  more  experimental  tests,  to  account  for  the 
change  in  the  character  of  the  season's  growth  of  wood. 
After  the  buds  open,  the  leaves  expand  and  grow,  the  inter- 
nodes  lengthen,  and  all  the  parts  and  their  component  cells 
attain  their  definitive  dimensions,  weights,  etc.  As  a  result, 
the  mechanical  strain  upon  the  parts  behind  increases.  It 
increases  not  only  with  the  weight  and  with  the  change  in 
position  of  the  weight,  which  produces  a  greater  leverage, 
but  also  with  occasional  sudden  and  often  very  great  addi- 
tions to  the  weight  by  wind  and  rain.  Meantime  there  is 
little  or  no  increased  demand  for  food,  and  transpiration, 
being  controllable  by  the  stomata,  is  not  likely  to  increase 
greatly.  There  is  ordinarily,  therefore,  no  great  addition  to 
the  conducting  system  (see  pp.  123-4). 

Strengthening  tissues — fibres,  tracheids,  thick-walled  ele- 
ments— are  formed  by  the  differentiation  of  the  young  cells 
derived  from  the  cambium.  From  Hegler's  investigations 
(p.  187 )  it  is  evident  that  strengthening  tissues  develop  ac- 
cording to  the  strain  to  which  a  part  is  subjected,  that  an 
increasing  strain  is  accompanied  by  the  formation  of  more 

*  Goff,  E.  S.  The  resumption  of  root-growth  in  spring.  Wisconsin 
Agric.  Exp.  Sta..  15th  Annual  Report  1898. 


IRRITABILITY  195 

and  stronger  mechanical  tissues,  and  that  this  development 
is  a  response  to  irritation.  We  have  only  to  apply  these 
conclusions  to  the  changes  taking  place  in  growing  wood  as 
the  season  progresses  to  gam  an  idea  as  to  one  of  the  most 
important  influences  contributing  to  the  formation  of  "au- 
tumn wood." 

Ordinarily  only  one  ring  is  added  to  the  wood  each  year. 
Many  woody  plants,  if  defoliated  by  frost,  caterpillars,  etc., 
so  early  in  the  season  that  growth  has  not  ceased,  will 
open  their  latent  buds,  and  develop  a  second  set  of  leaves. 
Under  these  conditions,  with  the  sudden  increase  in  the 
demand  for  new  conducting  tissues,  the  }roung  derivatives  of 
cambium  will  develop  into  elements  resembling  but  not 
quite  equalling  those  of  normal  "spring  wood."  In  this 
way  woody  plants  may  form  in  nature  two  rings  of  wood  in 
a  single  year.  It  is  claimed*  that  "spring"  and  "autumn" 
wood  may  be  found  repeatedly  alternating  with  one  another 
in  a  single  season's  growth  in  pine,  if  only  during  the  grow- 
ing season  there  are  repeatedly  alternating  and  sharply 
contrasting  rainy  and  dry  periods.  Pfeffer's  caution f  "that 
an  apparently  similar  result  may  sometimes  be  produced  in 
various  ways"  applies  to  this  observation  as  well  as  to 
experiments. 

The  careful  study  of  plants  subject  to  the  attack  of  gall- 
insects  and  other  pests  should  throw  light  on  the  relation 
of  the  growth  of  wood  to  the  demand  made  upon  it.  For 
example,  cross-sections  of  the  younger  branches  of  Monterey 
Pine  (Pinus  radiata)  which  have  been  attacked  by  the  leaf- 
galling  insect  Diplosis pirn-radiate ^%  show  abnormalities  in 
the  vascular  tissues.  Instead  of  the  clearly  marked  annual 
rings  of  wood,  these  branches  have  semi-annual  rings  which 
correspond  in  size,  position,  and  composition  with  the  times 
at  which  the  plant  is  attacked  by  the  gall-insect,  at  which 

*  Lutz,  K.  G.  Beitrage  zur  Physiologic  der  Holzgewachse.  Fiinfstiick's 
Beitrage  z.  wise.  Bot..  Bd.  I..  1897. 

t  Pfeffer,  W.  Pflanzenphysiologie.  2te  Aufl..  Bd.  II.,  p.  274,  1901. 

J  Papers  by  Cannon,  W.  A.  The  Gall  of  the  Monterey  Pine.  American 
Naturalist,  vol.  34.  1900;  and  Miss  Mills  in  Entomological  News,  vol. 
XI.,  1900. 


196  PLANT  PHYSIOLOGY 

the  morbid  growth  begins  and  ends,  and  with  the  differences 
in  the  activities  of  the  galled  leaves  from  those  which  are 
normal. 

The  formation  of  wood  and  of  its  different  kinds  and 
elements  in  a  season  is  affected  by  all  the  vital  activities 
of  the  plant  and  all  the  external  influences  which  bear 
upon  it.  The  character  of  the  wood  is  not  the  result  of 
any  one  set  of  factors.  At  the  same  time  that  we  must 
constantly  recognize  that  the  living  plant  is  sensitive  to  a 
great  many  influences  and  that  it  responds  to  these,  we  may 
distinguish  in  the  complex  of  influences  some  which  are  more 
effective  than  others.  We  may  therefore  accept,  at  least  un- 
til a  better  one  is  advanced,  this  hypothesis  :  the  two  kinds 
of  wood  in  the  year's  growth  are  formed  in  their  different 
ways  in  response  to  the  different  demands,  or  stimuli, 
brought  to  bear  upon  the  cambium  and  its  young  deriva- 
tives; the  "spring  wood,"  composed  of  large  elements,  essen- 
tially for  the  conduction  of  liquids;  the  "autumn  wood/' 
composed  of  small  and  thick-walled  elements,  essentially  for 
mechanical  strength;  and  between  these  two,  the  wood 
which  is  both  "spring"  and  "autumn"  in  character,  formed 
when  there  is  still  need  of  more  conducting  tissues  and  when 
the  need  of  strengthening  tissues  is  already  beginning.  So 
we  have  the  adaptation  of  the  wood  to  the  different  needs 
of  the  plant  in  different  parts  of  the  growing  season,  the 
adaptation  being  accomplished  through  the  irritability  of 
the  growing  and  differentiating  cells. 

INFLUENCE  OF  GRAVITATION. 

So  far  we  have  studied  mainly  the  effects  of  evidently 
mechanical  influences  upon  the  living  organism.  It  is,  how- 
ever, sensitive  to  many  other  influences,  some  of  them  quite 
as  important.  Of  these  only  one  is  constantly  and  uni- 
formly operative — the  force  of  gravitation.  The  plant  may 
change,  by  growth  and  movement,  the  relation  of  its  parts 
to  the  force,  but  the  amount  of  force  acting  upon  the  plant 
continues  the  same.  The  other  influences  are  variable, 
periodic,  or  occasional.  The  mechanical  influences  so  far 


IRRITABILITY  197 

discussed  may  be  said  to  control  growth  by  limiting  it. 
The  influences  which  we  are  about  to  study  control  growth 
by  directing  it.  Yet  this  distinction  is  suggestive  rather 
than  exact,  and  must  not  be  accepted  without  reserve. 

The  action  of  gravitation  may  be  considered  from  its 
effects  on  the  kind,  rate,  and  direction  of  growth,  and  on 
the  position,  of  plants.  Gravity  exerts  upon  all  objects  a 
pull  toward  the  centre  of  the  earth.  This  pull  is  propor- 
tioned in  intensity  to  the  weight  ( mass )  of  each  object,  the 
heavier  the  object  the  stronger  the  pull.  The  pull  is  resisted 
more  or  less  completely  by  the  medium  in  which  the  objects 
are.  An  object  in  a  fluid  is  buoyed  up  with  a  force  equal  to 
the  weight  of  the  fluid  which  it  displaces.  Thus  the  down- 
ward pull  of  gravity  upon  a  plant  living  submersed  in  fresh 
water  is  resisted  by  a  force  equal  to  the  weight  of  the  water 
which  the  plant  displaces.  This  is  750-800  times  greater 
than  the  force  which  an  equal  volume  of  air  would  exert. 
The  average  specific  gravity  of  sea- water  is  1.2.  Hence  a 
plant  living  in  sea- water  is  supported  still  more,  by  a  force 
1.2  times  greater  than  that  of  an  equal  volume  of  fresh 
water,  and  therefore  900-950  times  greater  than  air.  The 
parts  of  a  plant  growing  in  a  solid  medium,  the  soil,  are 
completely  supported.  The  soil  will  ordinarily  support 
much  more  than  the  weight  of  the  plants  growing  in  it.  It 
is  evident,  therefore,  other  things,  being  equal,  that  the 
mechanical  strength  which  the  plant  or  plant-part  must 
develop  is  proportioned  to  the  fraction  of  the  force  of 
gravitation  which  is  not  balanced  by  the  buoyancy  or  sup- 
porting power  of  the  medium  in  which  it  lives.  The  force  of 
gravity  exerts  by  this  means  a  direct  influence  upon  the 
kind  of  tissue  which  the  plant  forms,  the  kind  of  growth 
which  it  makes.  The  force  of  gravity  is  one  of  the  most 
important  factors  in  the  complex  which  constitutes  the 
environment. 

The  rate  of  growth  in  most  plants  seems  to  be  tolerably 
independent  of  gravity,  other  forces  being  more  effective. 
Parts  which  normally  stand  in  one  direction  may  grow  at 
a  somewhat  different  rate  when  their  position  is  changed. 
Also,  when  gravity  is  opposed  by  an  equal  or  greater  force, 


198  PLANT  PHYSIOLOGY 

as  can  be  done  by  a  centrifugal  machine,  the  rate  of  growth 
may  be  changed.  But  when  the  position  of  a  plant  is  con- 
stantly changed  or  is  changed  at  frequent  and  regular  inter- 
vals, as  by  a  regular  or  by  an  intermittent  clinostat,  the 
rate  of  growth  does  not  seem  to  be  materially  affected.* 

Gravity  is  one  of  the  most  important  influences  determin- 
ing the  direction  of  growth.  It  affects  direction  and  kind 
much,  the  rate  of  growth  little.  Its  action  in  directing 
growth  is  called  geotropism.  Those  organs  which  grow 
toward  the  source  of  gravitation,  downward,  are  said  to  be 
positively  geotropic.  Roots  and  rhizoids  are  positively 
geotropic  organs.  Stems  which  grow  in  the  opposite  direc- 
tion, upward,  are  called  negatively  geotropic.  Other  organs, 
such  as  branches,  which  grow  horizontally,  are  said  to  be 
diageotropic,  while  obliquely  growing  organs  like  lateral 
roots  are  called  plageotropic.  For  reasons  of  convenience, 
roots  have  long  been  the  favorite  objects  for  the  study  of 
the  effects  of  gravitation.  Such  horizontal  organs  as  leaves 
owe  their  position  quite  as  much  to  the  influence  of  light  as 
to  gravitation.  Though  we  may  say  that  gravitation  is  the 
chief  force  directing  the  growth  of  stems  and  roots,  it  must 
be  remembered  that  it  is  only  the  chief  of  several  or  many. 
The  position  which  any  part  or  organ  finally  assumes  repre- 
sents the  combined  influence  of  all  of  those  forces  to  which 
it  is  sensitive  and  which  act  upon  it. 

It  may  be  stated  as  a  general  rule  that  the  stems  and 
roots  of  higher  plants,  and  the  corresponding  parts  of 
many  lower  plants,  tend  to  grow  in  opposite  directions. 
Each  organism  begins  as  a  single  cell.  Upon  this  one  cell 
all  the  influences  which  affect  the  new  plant  are  converged. 
The  directions  of  growth  and  of  division  of  this  one  cell  are 
the  result  of  these  influences.  In  consequence  of  the  first  as 
well  as  of  subsequent  divisions,  the  different  cells  of  the 
embryo  are  differently  placed,  some  opposite  to  others. 
From  this  oppositeness  in  position  there  follows  an  oppo- 
siteness  in  behavior,  which  expresses  a  difference  in  the  cells 
and  organs  themselves.  It  is  easy  and  natural  to  suspect 

*  For  a  discussion  of  this  topic  see  Pfeffer's  Pflanzenphysiologie,  2te 
Aufl.,  Bd.  II.,  §  29,  1901. 


IRRITABILITY  199 

that  the  direction  of  the  divisions  of  the  fertilized  egg-cell 
and  of  the  first  cells  in  the  embryo  bears  a  definite  relation 
to  the  force  of  gravitation,  and  this  appears  in  many 
plants  to  be  really  the  case,  but  other  influences  may  co- 
operate or  predominate  in  producing  the  same  effect,  as, 
for  example,  in  the  ferns.*  Whatever  may  be  the  origin 
in  the  embryo  of  the  different  responses  which  the  differ- 
ent parts  make,  it  is  evident  that,  from  the  germination 
of  the  seed  onward,  the  plant  is  sensitive  to  gravitation 
and  is  directed  in  its  growth  by  it  as  well  as  by  other 
forces. 

The  responses  to  the  force  of  gravity  are  much  better 
known  than  are  the  immediate  effects  of  the  force  in  the 
sensitive  parts.  Most  of  our  knowledge  and  interest  in  the 
subject  are  due  to  Ciesielski,t  Darwin,!  Sachs,§  and  Pfeffer§§ 
and  their  followers. 

The  young  root  is  more  highly  differentiated,  anatomi- 
cally and  physiologically,  than  its  simple  external  appear- 
ance suggests.  At  the  extreme  tip  is  the  cap,  a  protective 
covering  of  the  "growing  point."  The  "growing  point"  is 
a  mass  of  permanent  meristem  which  gives  rise  to  all  the 
root  structures.  Behind  this  is  the  region  of  evident  growth, 
where  the  young  cells  are  increasing  in  volume  by  the  ab- 
sorption of  water  from  their  older  neighbors  (see  fig.  9, 
p.  168).  Still  further  back  and  adjoining  this  region  is  the 
zone  where,  through  the  root-hairs,  water  is  principally 
absorbed  from  the  soil.  If  a  young  seedling  with  a  straight 
root  be  laid  so  that  its  root  will  be  horizontal,  care  being 
taken  that  the  root  remain  moist,  the  tip  will  begin  to 

*  See  Goebel,  K.  Organographieder  Pflanzen,  pp.  188-f,  1898.  Campbell, 
D.  H.  Mosses  and  Ferns.  1895.  McMillan.  C.  The  orientation  of  the 
plant-egg  and  its  ecological  significance.  Botanical  Gazette,  vol.  25.  1898. 

f  Ciesielski.  T.  Untersuchungen  iiber  die  Abwartskrummiing  der  Wurzel. 
Beitrage  z.  Biologic  d.  Pflanzen.  Bd.  I.,  1872. 

\  Darwin.  C.    The  Power  of  Movement  in  Plants.  1880. 

§  Sachs,  J.  von.  Different  papers  from  1873  to  1879,  collected  in  his 
Gesa  melte  Abhandlungen  iiber  Pflanzenphysiologie.  Bd.  II.,  1893,  and  in 
his  Lectures  on  the  Physiology  of  Plants.  Eng.  transl..  1887. 

§§  Pfeffer,  W.  Geotropic  sensitiveness  of  the  root-tip.  Annals  of  Bcftany, 
vol.  VIII.,  1894. 


200  PLANT  PHYSIOLOGY 

turn  downward  within  an  hour  from  the  time  when  its  posi- 
tion was  changed.*  The  tip  is  carried  downward  by  the 
elongation  and  curvature  that  take  place  in  the  part  most 
rapidly  growing,  3-4  millimetres  back  of  the  tip.  In  this 
case  gravity  acts  as  a  stimulus.  Gravity  cannot  be  the  sole 
force  pulling  the  tip  into  the  soil,  for  the  tip  is  too  light, 
and  the  resistance  of  the  soil  is  too  great,  for  any  such 
result. 

Darwin  believed  that  the  tip  of  the  root,  like  the  brain,  is 
a  sense  organ,  receiving  the  stimulus  of  gravitation  and 
sending  back  to  the  elongating  part  tha  impulse  to  respond 
to  it.  Sachs  and  others  contended  that  only  the  growing 
part  received  the  stimulus  and  acted  upon  it.  For  years  the 
matter  stood  thus,  and  Kothert,t  very  carefully  reviewing 
the  whole  subject,  had  just  published  his  opinion  that  it 
could  not  be  decided  experimentally  when  Pfeffer  J  announced 
the  results  of  the  ingenious  experiments  conducted  under  his 
direction  by  Czapek.§  In  order  to  decide  the  matter  it  was 
necessary  to  employ  means  which  would  not  in  any  way 
injure  the  root  or  introduce  any  new  factors  into  the  experi- 
ment ;  decapitation,  wounding,  and  the  other  devices  re- 
sorted to  before  being  obviously  open  to  objections  serious 
enough  to  invalidate  the  conclusions  drawn  from  experi- 
ments involving  such  procedure.  This  was  accomplished  by 
taking  advantage  of  the  plasticity  of  the  growing  tip,  caus- 
ing the  roots  to  grow  into  tubes,  3-4  millimetres  long,  of 
thin  glass,  bent  in  the  middle  at  a  right  angle.  While  the 
tips  were  growing  into  these  tubes,  the  plants  were  revolved 
on  a  clinostat,  so  that  no  directive  geotropic  irritation 

*  For  experiments  on  roots,  seeds  may  best  be  germinated  in  damp 
tannin-free  sawdust.  (Stone.  Bot.  Gazette.  XIX..  1894.)  Many  experi- 
ments are  described  in  the  laboratory  manuals  previously  named. 

t  Rothert,  W.  Die  Streitfrage  iiber  die  Function  der  Wurzelspitze.  Flora. 
1894. 

t  Pfeffer,  W.  Cber  die  geotropische  Sensibilitat  der  Wurzelspitze.  Sit- 
zungsber.  d.  K.  Sachs.  Gesellsch.  d.  Wissensch.,  Sitzung  vom  2ten  Juli,  1894. 
Geotropic  sensitiveness  of  the  root-tip.  Annals  of  Botany,  vol.  VIII., 
1894. 

§  Czapek,  F.  Untersuchungen  iiber  Geotropismus.  Jahrb.  f.  wiss.  Bot., 
Bd.  XXVII..  1895. 


IRRITABILITY  201 

should  be  set  up.  "If  now  a  specimen,"  prepared  as  shown 
in  the  accompanying  figure,  "is  placed  so  that  the  terminal 
part  points  vertically  downwards,  whilst  the  rest  of  the  root 
is  horizontal,  no  geotropic  curvature  takes  place.  This, 
however,  always  took  place,  and  with  about  the  same 
promptness  as  in  straight  roots,  when  the  terminal  portion 
was  placed  horizontally,  or  in  general  at  an  acute  angle 
with  the  normal  position.  From  these  experiments  it  fol- 
lows that  the  root  thus  treated  is  perfectly  capable  of 
reaction.  .  .  .  By  this  means  therefore  it  is  proved  with 
the  most  perfect  certainty,  that  in  an  uninjured  root  only 
the  root-tip  is  geotropically  sensitive."  With  this  is  also 
proved  that  from  the 
part  which  is  sensitive 
—the  part  of  the  root  

^—j\  •"•         — 

with  the  largest  amount     (^ 

of  living  protoplasm  in 

proportion   to   its  vol- 

Fierure  14.    Root-tip  in  bent  glass  tube, 
ume-the   stimulus    is  (From  Czapek.} 

transmitted  to  cells  cap- 
able of  responding  to  the  stimulus.  These  are  the  cells 
at  that  time  increasing  in  volume,  growing.  The  posi- 
tion of  the  tip  is  determined  by  its  own  sensitiveness,  but 
its  position  can  be  changed,  except  by  artificial  means, 
only  by  the  action  of  the  responding  part.  The  stimulus  is 
transmitted  from  living  cell  to  living  cell.  The  transmission 
takes  time,  but  the  interval,  known  as  the  latent  period, 
between  exposure  and  response  to  the  stimulus,  is  necessarily 
employed  in  preparing  to  execute  the  response  as  well  as  in 
transmitting  the  stimulus.  The  latent  period  may  mean 
still  more,  but  at  least  it  means  these  two  things. 

The  transmission  of  the  stimulus  cannot  be  understood 
until  it  is  known  in  what  the  stimulus  exerted  by  gravity 
consists  and  what  it  does  in  the  sensitive  cells.  All  the 
ponderable  parts  of  an  organism  and  of  a  cell  are  subject 
to  the  physical  pull  of  gravity.  From  this  it  follows  that 
while  these  parts  are  in  place  their  position  is  maintained 
as  the  result  of  the  activity  of  living  protoplasm.  When  the 
position  of  these  parts  is  changed,  either  by  the  living  pro- 


202  PLANT  PHYSIOLOGY 

toplasm  or  by  means  outside  of  it,  the  relations  of  the  parts 
and  of  the  protoplasm  to  gravity  are  also  changed.  The 
force  of  gravity  acts  constantly.  Therefore,  whether  the 
ponderable  parts  of  a  cell  are  changed  or  are  constant  in 
position,  they  and  the  protoplasm  are  constantly  influenced 
by  gravity.  Only  when  there  is  a  change  in  the  relation  of 
the  parts  to  gravity  is  the  protoplasm  called  upon  to 
change  its  response.  The  action  of  gravity  in  a  cell  and 
upon  its  parts  must  be  the  same  as  upon  any  ponderable 
substance.  The  heavier  (or  lighter)  the  substance,  the 
greater  (or  less)  its  specific  weight  as  compared  with  the 
protoplasm  in  which  it  is  imbedded,  the  more  promptly 
will  it  exert  a  different  pressure  upon  the  protoplasm 
and  change  in  position  when  the  position  of  the  cell 
is  altered.  In  every  living  cell  there  are  solid  particles  of 
greater  or  less  size,  living  or  lifeless,  presumably  differing 
among  themselves  and  from  the  protoplasm  in  weight.  In 
certain  plants,  e.g.  some  Desmids,  in  rhizoids  of  Chara,  *  in 
the  starch-sheath  of  stems  and  in  the  root-cap,  etc.,  of  higher 
plants, f  solid  particles  are  conspicuous  in  size  or  arrange- 
ment. The  force  of  gravity  will,  however,  act  similarly, 
though  less  evidently,  upon  the  solid  contents  of  all  cells. 

The  reaction  of  the  living  protoplasm  to  the  pull  of 
gravity  is  very  different  from  that  of  lifeless  material.  The 
reaction  may  show  Itself  in  a  variety  of  ways,  chemical  and 
physical.  CzapekJ  has  shown  that  a  chemical  change  takes 
place  in  root  cells  stimulated  geotropically.  In  the  periblem 
cells  of  sensitive  root-tips  there  is  present,  in  larger  quantity 

*  Giesenhagen,  K.  Uber  inneie  Vorgange  bei  der  geotropischen  Kriim- 
mung  der  Wurzeln  von  Chara.  Ber.  d.  Deutsch.  Bot.  Gesellsch.,  XIX.,  1901. 

f  Haberlandt,  G.  Uber  die  Perception  des  geotropischen  Reizes.  Ber.  d. 
Deutsch.  Bot.  Gesellsch..  Bd.  XVIII.,  1900.  Uber  die  Statolithenfunction 
der  Starkekorner.  Ibid.,  Bd.  XX.,  1902.  Nemec,  B.  Uber  die  Art  der 
Wahrnehmung  des  Schwerkraftreizes  bei  den  Pflanzen.  Ibid.,  Bd.  XVIII., 
1900.  Uber  die  Wahrnehmung  des  Schwerkraftreizes  bei  den  Pflanzen. 
Jahrb.  f.  wiss.  Bot.,  Bd.  XXXVI..  1901.  Noll,  F.  Uber  Geotropismus. 
Ibid.,  Bd.  XXXIV..  1900.  Zur  Controverse  tiber  den  Geotropismus.  Ber. 
d.  Deutsch.  Bot.  Gesellsch.,  Bd.  XX.,  1902. 

f  Czapek,  F.  Ein  mikroskopischer  Befund  an  geotropisch  gereizten  Wur- 
zeln. Ber.  d.  Deutsch.  Bot.  Gesellsch.,  XV.,  1898.  Also  Jahrb.  f.  wiss.  Bot. 
Bd.  32,  1898. 


IRRITABILITY  203 

than  elsewhere,  a  readily  oxidized  substance,  perhaps  also 
another  which  is  a  vehicle  of  oxygen.  In  the  irritated  root- 
tip,  and  directly  in  proportion  to  the  irritation,  the  amount 
of  aromatic  oxidizable  substance  increases  and  the  substance 
or  substances  serving  as  a  vehicle  for  oxygen  decrease.  This 
transfer  of  oxygen  from  one  compound  to  another  within 
the  cell  is  accomplished  not  by  gravity  but  by  the  living 
protoplasm  stimulated  by  gravity. 

The  transmission  of  a  stimulus  which  caused  only  mechan- 
ical changes  in  the  stimulated  cell  would  be  very  difficult  to 
conceive.  On  the  contrary,  when  there  is  a  change  in  the 
amount  or  the  composition  of  diffusible  substances,  it  is 
much  easier  to  form  some  notion  of  the  means  by  which  the 
impulse  to  grow  in  a  definite  way  is  given  by  the  meriste- 
matic  cells  at  the  root-tip  to  those  in  the  growing  region 
behind.  Any  change  of  this  sort  in  one  cell  is  necessarily 
followed  by  corresponding  diffusion  currents  between  this 
cell  and  its  neighbors  more  and  more  remote.  Wherever  ST 
new  substance  enters  the  living  protoplasm  of  a  cell  it  will 
affect  the  protoplasm  chemically  or  physically,  thus  pro- 
ducing a  stimulus*  The  irritated  periblem  cells  of  the 
root-tip  give  up  by  diffusion,  the  changed  substances 
which  they  have  formed.  Finally  those  cells  in  the  grow- 
ing zone  are  reached  by  the  diffusing  compounds,/  and 
they  change  the  direction  if  not  also  the  rate  of  growth 
of  the  organ.  -^ 

Diffusion  undoubtedly  occurs  between  the  sensitive  cells  of 
the  root-tip  and  the  cells  more  or  less  remote.  The  trans- 
mission of  an  impul&e  by  diffusion  alone  is  not  rapid,  and 
it  is  difficult  to  prove.  To  avoid  both  of  these  objections 
Nemec*  has  recourse  to  protoplasmic  fibrils  to  which  he 
attributes  the  function  of  conducting  stimuli  from  cell  to 
cell.  These  fibrils  can  be  seen  in  suitably  prepared  fixed 
material  and  in  certain  living  cells  (e.g.  stamen  and  slime 
hairs  of  Tradescantia) ,  both  in  tissues  which  are  irritable 
and  which  exhibit  visible  reactions  to  stimuli >  and  also  in 
tissues  in  which  no  response  to  stimuli  has  ever  been  de- 

*  Nemec.  B.  Die  Reizleitung  und  die  reizleitenden  Structuren  bei  den 
Pflanzen.  Jena.  1901. 


204  PLANT  PHYSIOLOGY 

tected.*  Since  we  conceive  all  living  cells  to  be  affected  by 
external  influences,  whether  they  give  a  response  which 
can  be  seen  at  any  time  or  not,  there  is  every  reason  to 
believe  that  all  the  living  cells  in  a  body  are  connected  to- 
gether. Indeed,  Strasburger's  investigations  go  far  toward 
proving  that  such  is  really  the  case.f  By  Nemec's  fibrils  and 
Strasburger's  continuity  of  protoplasm  the  living  parts  of  a 
plant  are  united  in  one  system.  Granting  this  to  be  the 
case,  it  is  nevertheless  difficult  to  see  how  these  structures 
convey  a  purely  mechanical  stimulus  like  that  of  gravita- 
tion. Nemec's  fibrils  suggest  the  nerve  structures  in  ani- 
mals. Like  these  structures,  their  manner  of  working  is  a 
mystery.  We  may  therefore  cling  to  our  diffusion  hypothe- 
sis as  comprehensible,  if  difficult  of  proof,  leaving  the  future 
to  determine  the  value  and  the  functions  of  protoplasmic 
fibrils  and  protoplasmic  continuity. 

Sachs  asserted  that  when  a  root-tip  is  horizontal  it  is  in 
position  to  be  most  strongly  stimulated  by  gravity,  and 
that  then  the  curvature  of  the  growing  region  will  be  most 
rapid  and  most  pronounced.  Until  Czapek  J  determined  the 
necessary  duration  of  the  stimulus,  Sachs's  assertion  was  un- 
challenged. Czapek  ascertained  that  the  root-tips  of  vari- 
ous plants  require  from  fifteen  to  fifty  minutes'  exposure 
to  the  action  of  gravity  in  order  to  bend.  The  longer  the 
exposure,  the  more  pronounced  in  rapidity  and  angle  is  the 
bending.  Thus  roots  of  Lupinus  albus  will  bend  if  laid 
horizontal  for  twenty  minutes,  but  the  maximum  effect  will 
follow  an  exposure  of  four  hours.  When  the  root-tip  points 
upward  at  an  angle  of  45°,  not  when  it  is  horizontal,  the 
root  bends  most,  the  time  of  exposure  and  other  conditions 
being  the  same.  Above  this  angle,  the  spontaneous  growth- 
movements,  known  as  nutation  or  circumnutation,  interfere 

*  Haberlandt.  G.  Uber  Reizleitung  im  Pflanzenreich.  Biolog.  Central- 
blatt,  XXI..  1901.  tber  fibrillare  Plasmastructuren.  Ber.  d.  Deutsch.  Bot. 
Gesellsch.,  XIX.,  1902. 

f  Strasburger,  E.  fiber  Plasmaverbindungen  pflanzlicher  Zellen.  Jahrb. 
f.  wise.  Bot..  Bd.  36,  1901. 

J  Czapek,  F.  Untersuchungen  iiber  Geotropismus.  Jahrb.  f.  wise.  Bot.. 
Bd.  27,  1895.  Weitere  Beitrage  zur  Kenntniss  der  geotropischen  Reizbewe- 
gungen.  Jahrb.  f.  wise.  Bot.,  Bd.  32  1898. 


IRRITABILITY  205 

with  the  action  of  gravity.    Czapek's   conclusion   is   sup- 
ported by  a  study  of  grass-haulms.* 

When  the  root-tip  is  stimulated  by  the  action  of  gravity  a 
bending  takes  place  further  back,  in  the  region  which  is  most 
rapidly  elongating.  This  region,  called  the  motor  zone,  can 
bend  only  if  there  are  differences  in  the  rate  of  growth  or 
in  the  tissue  tensions  in  its  different  parts.  It  is  not  neces- 
sarily the  case  that  all  roots  behave  alike  in  the  means 
any  more  than  hi  the  manner  of  curvature,  although  it 
seems  probable  that  in  roots  of  similar  structure  the  me- 
chanics of  curvature  will  be  similar.  This  may  account  hi 
part  for  the  diverse  views  regarding  the  mechanics  of  geo- 
tropic  curvature.  Thus,  according  to  Ciesielski,  f  the  cells  of 
the  lower  (concave)  side  of  the  root  are  forcibly  com- 
pressed, even  wrinkled,  by  the  more  than  average  growth  of 
the  cells  of  the  upper  ( convex )  side.  Sachs  J  denies  that  the 
cells  of  the  upper  side  always  grow  faster,  and  attributes 
the  pronounced  bending  to  the  diminished  growth  rate  of 
the  cells  of  the  lower  side.  MacDougal,§  employing  the 
imbedding  methods  now  in  general  use  but  unknown  to 
Sachs  when  he  wrote  on  this  subject,  comes  to  essentially 
the  same  conclusions  as  the  great  master  hi  plant  physi- 
ology more  than  twenty-five  years  before.  Pollock,!  study- 
ing curvatures  induced  by  injury  ( traumatropic )  instead  of 
by  gravity  (geotropic),  concludes  that  the  mechanism  of 
root-curvature  consists  in  changed  tissue-tensions  in  the 
stimulated  roots,  the  normal  tension  between  cortical  paren- 
chyma and  axial  cylinder  increasing  on  the  upper  ( convex ) 
side  and  decreasing  on  the  lower  (concave)  side.  Though 
this  last  view  may  be  correct  as  regards  some  if  not  all 
roots,  it  must  be  conceded  that  growth  makes  permanent 

*  Pertz.  Miss  D.  F.  M.  On  the  gravitation  stimulus  in  relation  to  posi- 
tion. Annals  of  Bot.,  XIII.,  p.  62<X  1899. 

t  Ciesielski.  Th.  Untersuchungen  iiber  die  Abwartskrummung  der  Wur- 
zel.  Conn's  Beitr.  z.  Biol.  d.  Pflanzen.  Bd.  I..  1871. 

J  Sachs,  J.  von.    Gesammelte  Abhandlungen,  Bd.  II..  p.  852. 

§  MacDougal.  D.  T.  The  Curvature  of  Roots.  Bot.  Gazette,  vol.  23^ 
1897. 

•"  Pollock,  J.  B.  Mechanism  of  root-curvature.  Bot.  Gazette,  vol.  29,, 
1900. 


206  PLANT  PHYSIOLOGY 

the  changes  induced  by  irritation,  whether  these  changes  in 
direction  are  first  effected  by  changes  in  the  tissue-tensions 
of  the  motor  zone  or  by  growth  itself. 

What  has  been  said  about  the  irritability  and  means  of 
response  of  roots  which  grow  vertically  ( orthotropic ) ,  ap- 
plies equally  to  lateral  roots  which  grow  obliquely  ( plagio- 
tropic).  The  difference  between  these  organs  lies  in  their 
different  relations  to  irritation  by  gravity.  They  may  not 
differ  in  sensitiveness,  but  they  will  respond  in  different 
degrees  to  the  same  force.  We  may  conceive  this  to  be  due 
to  the  different  balances  of  all  the  influences  to  which  the 
two  kinds  of  roots  are  subject.  If  both  lateral  and  pri- 
mary roots  grew  vertically,  they  would  interfere  writh  one 
another  in  position  and  in  functions,  failing  to  attain  their 
utmost  usefulness  and  irritating  one  another  by  their  prod- 
ucts. This  may  be  the  main  factor  in  determining  the  po- 
sition of  lateral  in  relation  to  primary  roots.  The  response 
of  lateral  roots  to  the  stimulus  of  gravity  will  be  changed 
when  the  primary  root  is  removed  or  so  injured  that  its 
further  vertical  growth  is  impossible.  The  lateral  roots  will 
then  bend  down  and  one  or  more  will  attempt  to  take  the 
place  in  position,  direction  of  growth,  etc.,  of  the  primary 
root.  The  relation  of  any  organ  to  any  one  influence  or 
combination  of  influences  depends  upon  the  relation  of  all 
the  other  organs  to  each  one  of  the  influences  composing 
the  total  to  which  the  organism  is  subject.* 

Among  stems  as  among  roots  there  are  erect  (ortho- 
tropic)  and  horizontal  ( plagiotropic )  organs,  differing 
from  one  another,  not  in  their  sensitiveness  or  in  the  effi- 
ciency of  their  response  to  gravity,  but  in  their  relation  to 
the  force.  In  the  ordinary  orthotropic  stem,  unlike  the  root, 
there  is  no  separation  into  sensory  and  motor  zones,  though 
Francis  Darwin  f  has  shown,  in  the  seedlings  of  certain 
grasses,  that  the  first  leaf-sheath  is  sensitive  to  gravitation 
and  the  bending  is  accomplished  only  by  the  segment  of 

*  Consult  Czapek,  loc.  cit.,  and  Schober,  Das  Verhalten  der  Neben- 
wurzeln  in  der  verticalen  Lage.  Bot.  Zeitung,  1898. 

f  Darwin,  F.  On  geotropism  and  the  localization  of  the  sensitive  re- 
gion. Annal*  of  Bot.,  XIII.,  1899. 


IRRITABILITY  207 

stem  immediately  below.  In  ordinary  ortnotropic  stems  geo- 
|  tropic  sensitiveness  and  response  are  possible  only  in  elon- 
gating parts,  and  since  the  nodes  and  internodes  soon  cease 
to  grow  in  length,  geotropic  phenomena  soon  cease.  In 
such  stems  the  cortical  parenchyma,  uniform  in  structure 
and  usually  containing  chlorophyll,  is  the  sensitive  tissue, 
all  the  other  tissues  cooperating  in  the  response  to  the 
stimulus.  In  grass  haulms,  on  the  other  hand,  there  being 
a  persistent  meristem  at  the  base  of  each  internode  and 
in  this  no  visible  differences  among  the  cells,  we  may  as- 
sume that  the  whole  meristem  is  sensitive  as  well  as  re- 
sponsive. When  a  grass  haulm  is  laid  prostrate  by  wind 
or  rain,  the  meristem  cells  on  the  lower  side  of  each  inter- 
node  begin  again  to  divide,  growth  is  resumed,  and  by  this 
means  the  parts  nearer  the  tip  are  gradually  carried  again 
into  the  vertical  position. 

Prostrate  and  underground  stems  and  branches,  and  es- 
pecially leaves,  though  sensitive  to  gravity  and  assuming 
very  definite  positions  when  acted  upon  by  gravity  alone,  are 
so  much  more  strongly  affected  by  light  that  their  position, 
as  well  as  their  form,  size,  and  structure,  must  be  ascribed 
mainly  to  its  influence  rather  than  to  gravity  (see  pp.  214- 
215)." 

After  the  growing  (motor)  zone  has  executed  the  response 
to  the  stimulus  received  by  the  organ,  the  whole  organ 
returns,  like  the  sense  organs  and  muscles  of  an  animal,  to 
that  state  of  delicate  balance  and  readiness  in  which  the 
first  stimulus  found  it.  If  such  a  return  is  made  impossible 
by  continued  stimulation,  the  response  will  be  continuous 
until  fatigue,  insensibility,  and  impotence  are  produced,  or 
until  the  response  carries  the  organ  into  such  a  position 
that  other  influences  will  modify  or  prevent  any  further  re- 
sponse. This  has  been  shown  by  Frank,*  Darwin,f  and 
Copeland,  t  who  found  that,  on  confining  the  sensitive  part 

*  Frank.  A.  B.  Beitrage  zur  Pflanzenphysiologie  :  I.  t!ber  die  durch  die 
Schwerkraft  verursachten  Bewegungen  von  Pflanzentheilen.  Leipzig,  1868. 

t  Darwin.  F.  On  geotropism  and  the  localization  of  the  sensitive  re- 
gion. Annals  of  Bot..  XIII..  1899. 

t  Copeland.  E.  B.  Studies  on  the  geotropism  of  stems.  Bot.  Gazette, 
XXIX.,  1900. 


208  PLANT  PHYSIOLOGY 

to  a  horizontal  position,  an  almost  continuous  bending  takes 
place.  Similar  to  this  is  Elfving's  observation*  that  when  a 
fully  grown  grass  internode  is  revolved  horizontally  on  a 
clinostat,  its  meristem  resumes  its  activity,  its  cells  divide 
uniformly  under  the  stimulus  of  gravity  applied  successively 
to  all  its  parts,  and  the  internode  passes  through  another 
period  of  growth  in  length. 

The  force  which  a  geotropically  bending  organ  exerts  is 
manifestly  considerable,  e.g.  that  employed  in  restoring  a 
prostrate  grass  haulm  to  the  vertical  position.  The  force 
thus  exerted  is  developed  by  the  growing  and  bending  part, 
and  we  have  seen  (p.  176)  that  growing  organs  may  exert 
force  equalling  a  pressure  of  ten  atmospheres.  As  in  ordi- 
nary growth,  stems  and  roots  bending  because  of  geotropic 
stimulation  will  develop  only  the  amount  of  force  needed  to 
accomplish  the  bending.  This  has  been  proved  experiment- 
ally by  Meischke.t  He  ascertained  the  force  ordinarily 
exerted  by  geotropically  bending  parts,  and  found  that, 
when  compelled  to  do  so,  they  developed  from  several  to 
many  times  this  force  in  order  to  accomplish  the  bending. 
Thus— 

Grass  internodes  can  develop     4  times  the  ordinary  bending  force. 
Cucurbita  seedlings  13     " 

Lupinus  17      " 

Phaseolus  28     ' 

Helianthus        "  30     ' 

If  any  evidence  of  geotropic  stimulus  were  needed,  more 
striking  than  the  geotropic  curve  itself,  these  figures,  in- 
dicating the  increase  in  power  under  stimulus,  would  fur- 
nish it. 

INFLUENCE  OF  LIGHT 

Light  as  a  form  of  energy  does  certain  kinds  of  work  and 
accomplishes  certain  chemical  changes  within  as  well  as  out- 
side living  cells.  Living  protoplasm  is  composed  of  chemi- 

*  Elfving.  F.  Verhalten  der  Grasknoten  am  Klinostat.  Ofversigt  af 
finska  Vet.  Soc.,  Forhandlinger,  Bd.  26,  1884. 

t  Meischke.  P.  Uber  die  Arbeit sleistung  der  Pflanzen  bei  der  geotro- 
pischen  Kriimmung.  Jahrb.  f.  wiss.  Bot..  Bd.  33,  1899. 


IRRITABILITY  209 

cally  unstable  substances.  Some  of  its  constituents  and 
organs  bear  also  definite  physical  relations  to  light.  Light 
is  neither  constant  nor  uniform  in  amount,  its  direction  and 
intensity  change,  hence  its  action  upon  the  same  organ  or 
organism  is  different,  may  even  be  opposite,  at  different 
times.  Light  affects  the  living  protoplasm  through  the 
chemical  processes  and  physical  conditions  of  the  cell 
at  all  influenced  by  light.  Sunlight  and  the  ordinary  ar- 
tificial lights  are  composed  of  rays  of  different  sorts  which 
separately  affect  the  substances,  living  and  lifeless,  exposed 
to  them.  The  effect  of  ordinary  white  light  is  then  the  sum 
of  the  effects  of  its  constituent  rays.  These  rays,  visible  and 
invisible,  fall  into  three  classes,  the  thermal  or  heat,  the 
luminous  or  light,  and  the  actinic  or  chemical  rays.  Their 
effects  are  partly  suggested  by  their  names.  The  luminous 
rays,  being  the  ones  most  concerned  in  food-manufacture, 
affect  the  protoplasm  in  ways  which  we  have  already  studied 
(see  pp.  53-57).  The  heat  rays  we  have  also  examined  in 
their  relation  to  transpiration,  etc.  (pp.  136-151),  and  we 
shall  study  them  further  in  the  succeeding  section  of  this 
chapter  (pp.  219-222).  We  have  here  to  consider  mainly 
the  influence  of  the  visible  and  invisible  (" ultra-violet") 
chemical  rays. 

Some  idea  as  to  why  and  how  actinic  rays  affect  living 
protoplasm  so  strongly— in  other  words,  why  living  proto- 
plasm is  so  very  sensitive  to  light — may  be  gained  by  con- 
sidering how  powerfully  those  rays  influence  lifeless  sub- 
stances and  many  processes  taking  place  under  the  more 
readily  understood  conditions  prevailing  outside  the  living 
organism.  For  example,  *  olive  oil  oxidizes  when  exposed  to 
light,  oxalic  acid  in  solution  will  break  up  into  carbon- 
dioxide  and  formic  acid  in  the  light,  alcoholic  solutions  of 
chlorophyll  decompose  more  rapidly  in  light  than  in  dark- 
ness, and  many  enzyms  are  destroyed  by  exposure  to  light. 
These  are  all  organic  compounds  occurring  in  plant  cells. 
The  most  significant  in  the  list  are  the  enzyms.  Enzyms 
accomplish  the  conversion  of  insoluble  substances  in  the 

*  Davenport.  C.  B.  Experimental  Morphology.  Part  I.,  pp.  162-5, 
1897. 

14 


210 


PLANT  PHYSIOLOGY 


cell  into  soluble,  portable,  and  useable  compounds  (see 
pp.  29-30).  If  the  work  of  the  enzyms  is  interfered  with 
by  light,  the  functions  of  the  cell  must  be  altered  or  become 
deranged.  The  substances  composing  the  living  protoplasm 
may  also  be  as  sensitive  to  light  as  its  products.  The  sensi- 
tiveness to  light  of  the  com- 
pounds in  and  perhaps  compos- 
ing their  cells  is  the  reason  for 
the  sensitiveness  of  living  organ- 
isms to  light. 

The  influence  of  light,  like  the 
influence  of  gravity,  may  show 
itself  in  the  kind,  rate,  and  direc- 
tion of  growth,  and  in  the  posi- 
tion of  the  organ  and  organism. 
Plants  kept  in  darkness  will  be 
longer  than  plants  under  nor- 
mal illumination.  Seeds  sprouted 
in  darkness,  and  seedlings  grow- 
ing where  no  light  falls  upon 
them,  grow  under  the  influences 
of  all  other  forces  than  light. 
When  the  influence  of  light  is 
wholly  eliminated,  root,  stems, 
and  branches  grow  longer  but 
are  proportionally  more  slender 
than  in  ordinary  sunlight,  the 
leaves  are  smaller  and  weaker, 
flowers  do  not  form  (see  pp. 
271-4).  That  stems  are  more 
slender  and  mechanically  weak- 
er in  darkness  than  in  light 
may  be  due  to  the  less  than  nor- 
mal weight  of  the  small  weak  leaves — to  the  absence  of  me- 
chanical strain  (see  p.  188).  The  leaves  require  a  certain 
amount  of  light  in  order  properly  to  develop  the  food- 
manufacturing  tissues.  Yet,  comparing  the  total  growth  of 
plants  of  the  same  species  in  light  and  in  darkness,  it  will 
be  clear  that  the  growth  is  greater  in  darkness  so  long  as 


Figure  15.  Branch  of  Cactus, 
the  young  parts  of  which  grew 
in  darkness.  (From  Goebel.) 


IRRITABILITY  211 

the  plant  is  adequately  nourished.  Light  retards  the 
growth  of  those  organs  which  do  not  depend  upon  it  for 
energy  for  food-manufacture. 

The  form  as  well  as  the  size  of  parts  is  greatly  influenced 
by  light.  If  a  branch  of  cactus,  Opuntia,  be  potted  and 
kept  in  darkness,  the  buds  will  develop  into  branches  which 
are  small  and  cylindrical,  while  the  branches  formed  in  the 
light  are  larger  and  flat.*  Marchantia ' gemmse,  germinated 
in  the  light  but  on  a  clinostat,  develop  in  two  months  into 
small  erect  tubular  plants,  wholly  unlike  the  flat  expanded 
thalli  which  form  when  light  and  gravity  act  normally.! 

The  daily  periodicity  of  light  and  darkness  are  almost 
coincident  with  the  daily  periodicity  in  growth-rate.  This 
was  first  shown  by  Sachs,  J  who  devised  machines  (auxan- 
ometers )  for  autographically  recording  the  growth  of  plants 
during  extended  periods.  These  records  show  that  the  rate 
of  growth  in  length,  of  plants  furnished  with  all  the  food 
they  need,  will  reach  its  maximum  about  sunrise,  its  mini- 
mum about  sunset.  That  growth  is  not  most  rapid  when 
there  is  least  light,  or  slowest  when  the  light  is  strongest, 
shows  that  the  effect  of  light,  like  that  of  gravity,  is  cumu- 
lative (p.  204).  Maximum  growth-rate  and  minimum 
temperature  almost  coincided  in  Sachs's  experiments,  but 
this  indicates,  not  that  coolness  favors  growth,  but  that 
Sachs  did  not  make  his  experiments  at  constant  tempera- 
tures. Sachs's  experiments,  repeated  at  constant  tempera- 
tures, show  that  the  maximum  and  minimum  growth-rates 
occur  at  approximately  the  same  times  as  before,  and  that 
light  and  not  heat  is  the  predominant  influence  in  con- 
trolling the  rate  of  growth. 

It  is  claimed  that  light  favors  the  growth  of  the  vegeta- 
tive organs  of  submersed  aquatics.  A  certain  minimum 

*  Vochting,  H.  Uber  die  Bedeutung  des  Lichtes  fiir  die  Gestaltung 
blattformiger  Kakteen.  Jahrb.  f.  wise.  Bot.,  Bd.  26,  1894.  Goebel,  K. 
Organographie,  I.,  pp.  211+,  1898.  Also  Flora.  Bd.  80,  1895. 

f  Czapek,  F.    7.  c.,  1898.  p.  261.  271,  etc. 

J  Sachs,  J.  von.  Uber  den  Einfluss  der  Lufttemperatur  und  des  Tag- 
eslichtes  auf  die  stiindlichen  und  taglichen  Anderungen  des  Langenwachs- 
thums  (Streckung)  der  Internodien.  Arb.  d.  Bot.  Inst.,  Wiirzburg,  1872. 
Gesamirelte  Abhandlungen,  Bd.  II..  1893. 


212 


PLANT  PHYSIOLOGY 


amount  of  light  is  required  by  them  to  carry  on  the  essen- 
tial process  of  food-manufacture.  A  larger  amount  acts  as 
a  stimulus  to  the  formation  of  reproductive  organs  (pp. 
264-8 ) .  Otherwise  the  relations  of  these  plants  to  light  are 
similar  to  those  of  land  plants.  Since  the  water  and  the 
materials  at  the  bottom  of  a  natural  body  of  water  absorb 


0 

1000 

[umber  per  c.c. 
2000 

3000 

4C 

00 

o 

2 

In'te 
0        4 

nsity  of  lig 
0         60 

lit 
80       1 

"&*+ 

2 

-^ 

4 

^' 

f 

6 

•y 

/ 

1 

8 

J 

FIGURE  16.    DIAGRAM  SHOWING  THE  GROWTH  OF 
DIATOMS  AND  THE  INTENSITY  OF  LIGHT 

,s 

1 

0 

1 

3 

2 

III 

AT  VARIOUS  DEPTHS. 

Water  in  Lake  Cochituate,   Massachusetts,  Nov.  29, 
1895.    Examined  Dec.   9,    1895.     Temperature  40-44°. 
Color  0.33  (pt.  standard)  .     Diatom  schiefly  Asterion- 
ella  and  Melosira.    Intensity  of  light  at  different  depths 
calculated  on  assumption  that  layer  of  aq.  1  ft.  in  depth 
absorbs  35  per  cent,  of  light  falling  upon  it. 
(AfterWhipple). 

4 

i/i 

6 

8 

10 

—  —  _ 

4 

light,  plants  living  in  such  a  situation  receive  less  light  than 
the  plants  living  on  the  bank  near  by.  Plants  living  on  or 
over  a  dark  or  black  bottom  receive  light  only  or  mainly 
from  almost  vertically  above.  The  quality  as  well  as  the 
quantity  of  light  reaching  water  plants  differs  from  that 
falling  upon  plants  in  the  air.  Water  absorbs  the  different 
kinds  of  rays  unequally,  the  luminous  rays  least,  the  actinic 
rays  more.  Whipple*  cultivated  diatoms  in  bottles  at  dif- 

*  Whipple,  G.  C.  Some  experiments  upon  the  growth  of  diatoms.  Tech- 
nology Quarterly,  vol.  IX.,  1896.  Also  Microscopy  of  Drinking  Water, 
1901. 


IRRITABILITY  213 

ferent  depths  in  a  reservoir.  Taking  the  numbers  present 
as  an  index  of  the  suitableness  of  the  illumination  at  differ- 
ent depths — a  standard  not  above  criticism — the  largest 
number,  and  inferably  the  best  illumination,  are  found  a 
few  inches  below  the  surface.  The  accompanying  diagram 
indicates  the  results  of  the  experiment.  On  the  surface, 
where  there  was  no  absorption  of  light,  and  where  the 
organisms  could  therefore  receive  most  light,  the  growth  ( 7.^. 
the  reproduction )  was  less  than  when  a  thin  layer  of  water 
absorbed  the  actinic  rays. 

Certain  seeds  and  spores  appear  to  germinate  better,  and 
perhaps  only,  in  light.  *  The  European  mistletoe  (  Viscum 
,'ilbuin),  the  seeds  of  some  grasses,  and  the  spores  of  some 
of  the  vascular  cryptogams  are  said  not  to  germinate  in 
darkness.  In  these  cases  we  have  the  stimulating  influence 
of  light  upon  processes  which  precede  growth  ( e.g.  respira- 
tionf ) .  After  their  germination,  the  influence  of  light  upon 
growth  is  the  same  as  in  other  plants. 

The  influence  of  light  upon  the  rate  of  growth  is  evident 
from  the  foregoing;  light  lowers  the  growth-rate.  If  un- 
equal amounts  of  light  fall  on  different  parts  of  a  growing 
organ,  the  growth  of  the  organ  will  be  unequal.  By  this 
means  the  direction  of  growth  of  an  organ  or  organism  is 
influenced  by  light.  The  stems  of  plants  growing  indoors 
near  a  window  turn  toward  the  light,  the  leaves  spread 
themselves  at  right  angles  to  the  incident  rays,  the  roots 
(when  visible  at  all)  turn  away  from  the  light.  The  be- 
havior of  the  parts  of  such  plants  is  readily  accounted  for 
on  the  basis  of  what  has  already  been  said.  The  growing- 
cells  on  the  side  of  the  stem  away  from  the  window  receive 
less  light  and  are  less  checked  in  growth  than  those  on  the 
opposite  side,  and  hence  push  the  tip  of  the  stem  over  to- 
ward the  window.  The  influence  of  light  upon  the  direction 
of  growth,  though  we  see  that  it  is  due  to  the  effect  of  light 
on  the  rate  of  growth,  is  known  as  heliotropism  or  photo- 
tropism. 

*  Davenport,  C.  B.  Experimental  Morphology,  Part  II.,  pp.  423  -  5, 1899. 
f  Kolkwitz.   R.    t^ber  den    Einfluss  des  Lichtes    auf   die  Athmung   der 
niederen  Pilzen.    Jahrb.  f.  wise.  Bot.,  Bd.  33.  1898. 


214  PLANT  PHYSIOLOGY 

Most  stems  are  positively  heliotropic,  that  is,  bend  toward 
light  of  ordinary  intensity  and  composition.  The  English 
Ivy  (Hedera),  to  a  certain  extent  Ampelopsis,  the  touch- 
me-not  (Impatiens),  the  garden  nasturtium  in  its  older 
stages  (Tropceolum  majus),*  and  some  other  plants,  are 
negatively  heliotropic  toward  intense  light,  though  not  to 
light  of  lower  intensity.  These  are  plants  which  either 
climb  against  objects  which  absorb  much  light  and  hence 
appear  dark,  or  thrive  best  in  comparative  shade.  The 
behavior  of  the  roots  which  attach  these  climbers  to  their 
supports  is  consistent  with  the  relation  of  most  roots  to 
light. 

Roots  are  not  markedly  sensitive  to  light,  light  serving, 
as  a  rule,  only  to  supplement  gravity.  So  much  more 
does  gravity  influence  the  direction  of  growth  of  roots  that 
the  influence  of  light  is  scarcely  apparent!  until  all  parts 
are  uniformly  subjected  to  gravitation  by  means  of  the 
elinostat. 

Underground  and  other  horizontally  growing  stems  exhibit 
a  peculiar  adjustment  to  the  influences  of  gravity  and  light, 
their  position  being  the  resultant  of  their  reactions  to  these 
two  opposite  stimuli.  The  prostrate  shoots  of  Rubus,  J  the 
runners  of  strawberry,  etc.,  and  the  root-stocks  of  Nuphar§ 
are  examples.  Goebel  shows  that  the  root-stock  of  Nuphar 
luteum  creeps  horizontally  in  the  mud,  possessing  at  this 
time  a  somewhat  dorsi-ventral  structure.  If  covered  with 
soil,  such  a  rhizom  would  bend,  grow  vertically  upward 
until  it  came  to  the  light,  the  structure  of  the  vertical  part 
being  radial.  At  the  surface  of  the  soil  it  would  bend  to 
the  horizontal  and  grow  through  the  muddy  twilight  as 
before. 

The  position  and  direction  of  growth  of  leaves  is  also  a 
resultant  of  the  two  forces,  gravity  and  light.  If  a  plant  is 

*  Wiesner,  J.  Die  heliotropischen  Erscheinungen.  Denkschrift  der  Wiener 
Akadernie,  Bd.  39  u.  43,  1878  u.  1880. 

f  Wiesner,  J.    /.  o. 

$  Czapek,  Fr.    7.  c.  1898,  p.  245,  257,  etc. 

§  Goebel,  K.  Organographie  der  Pflanzen,  I.,  p.  198,  1898.  Stahl,  E. 
Einfluss  des  Lichtes  auf  den  Geotropiemus  einiger  Pflanzenorgane.  Ber.  d. 
Deutsch.  Bot.  Gesellsch.,  Bd.  II..  1884. 


IRRITABILITY  215 

revolved  horizontally  around  its  long  axis  on  a  clinostat, 
thus  exposing  it  uniformly  on  all  sides  to  gravity  and  to 
light,  its  leaves  will  assume  no  uniform  and  definite  posi- 
tion. However,  when  the  plant  is  illuminated  from  one  di- 
rection only  while  it  is  being  revolved  on  the  clinostat,  the 
leaves  will  take  a  very  definite  position,  such  that  the  blades 
are  at  right  angles  to  the  incident  rays. 

Light  and  gravity  work  very  closely  together  in  determin- 
ing the  direction,  size,  and  even  form  into  which  organs 
and  organisms  grow.  When  one  influence  is  reduced  or  ex- 
cluded, e.g.  light,  there  is  a  change  in  the  organism  in  its 
relations  not  only  to  the  influence  excluded  but  also  to 
those  that  remain.  The  influence  of  the  one  force  or  the 
other  may  predominate  in  one  set  of  organs — as  the  roots 
are  affected  mainly  by  gravity,  the  leaves  mainly  by  light — 
or  they  may  be  nearly  or  quite  balanced.  Profoundly  as 
they  affect  the  parts  sensitive  to  them,  these  forces  are 
only  factors  in  the  complex  of  influences,  mechanical,  nutri- 
ent, and  other,  which  together  determine  the  character  of 
the  organism.  No  figures  as  to  the  relative  effectiveness  of 
gravity  and  light  acting  upon  the  whole  organism  can  be 
given,  yet  Czapek*  presents  some  figures  of  the  same  objects 
under  like  conditions,  indicating  the  minimum  times  of  ex- 
posure for  reactions  to  gravity  and  light. 

GEOTROP.  HELIOTROP. 

Avena  sativa,  ( etiolated )  15  minutes     7  minutes 

Sinapis  alba,  hypocotyl  (etiolated)  15  "  10  " 

Beta  vulgaris,                           "  15  "  10  " 

Zea  Mais,  primary  root  20  " 

Helianthus  annuus,  hypocotyl  20  ".  20  " 

Phaseolus  multiflorus,  epicotyl  50  "  50  " 

Phy corny ces  nitens,  sporangiophore  15  "  7 

The  direction  of  locomotion  in  motile  plants  is  influenced 
more  by  light  than  by  any  other  one  influence  except  that 
of  food  or  of  other  stimulating  compounds.  An  organism 

*  Czapek,  F.  Weitere  Beitrage  zur  Kenntniss  der  geotropischen  Reiz- 
bewegungen.  Jahrb.  f.  wiss.  Bot..  Bd.  32,  p.  185,  1898. 


216  PLANT  PHYSIOLOGY 

able  to  move  about  will  finally  reach  the  position  in  which 
it  is  most  favorably  affected  by  all  the  influences  to  which 
it  is  susceptible.  Other  positions  might  be  more  favorable 
for  separate  influences,  but,  the  influences  continuing  con- 
stant, the  position  occupied  may  be  regarded  as  the  best. 
If  the  influences  remained  constant  we  could  conceive  that 
the  sensitive  motile  organism,  which  had  come  into  the 
place  and  position  in  which  the  balance  of  stimuli  was  most 
favorable,  would  never  move  again.  The  influences  are  all 
changing,  except  gravity,  and  hence  the  organism  is  exposed 
to  changing  degrees  and  kinds  of  stimulation.  We  may 
imagine  that  the  movements  of  freely  motile  organisms 
indicate  three  things — the  rate  at  which  the  stimuli  change, 
the  degree  of  sensitiveness,  and  the  rate  of  response  of  the 
organism.  The  stimuli  may  change  very  rapidly,. but  a  dull 
or  feeble  organism  will  not  move  correspondingly  fast. 
Conversely,  an  extremely  sensitive  and  strong  organism  will 
be  in  rapid  motion  even  when  conditions  are  in  the  main 
unchanged.  But  since  conditions  are  not  absolutely  identi- 
cal at  two  different  points,  locomotion  itself  introduces  the 
organism  to  new  stimuli. 

Diatoms,  desmids,  Oscillatoria,  swarm-spores,  ordinary 
bacteria,  sulphur  bacteria  ( both  the  red  and  the  colorless ) , 
plasmodia,  and  many  other  motile  organisms,  move  toward 
light  until  they  reach  the  point  of  optimum  illumination. 
Locomotion  directed  by  light  is  called  phototaxis  or  helio- 
taxis.  Movement  toward  the  light  is  said  to  be  positive 
phototaxis,  away  from  the  light  negative  phototaxis.  Most 
organisms  are  both  positively  and  negatively  phototactic, 
depending  upon  the  degree  of  illumination  to  which  they  are 
exposed.  Thus  the  various  species  of  red  sulphur  bacteria 
now  growing  in  my  laboratory  collect  on  the  brightest 
parts  of  the  jars  in  which  they  are  living  when  the  jars  are 
on  a  table  six  feet  from  the  nearest  window,  and  on  the  less 
or  least  brightly  lighted  parts  when  the  jars  are  on  the 
window-sills. 

Where  locomotion,  or  at  least  a  change  in  position  of  the 
whole  organ  or  organism,  is  impossible,  the  organs  of  the 
cell  are,  in  many  cases,  able  to  move  in  accordance  with  the 


IRRITABILITY 


217 


intensity  and  direction  of  the  light.  This  is  shown  most 
clearly  by  the  movements  of  chromatophores.  *  The  ac- 
companying figures  a,  b,  c,  were  drawn  on  a  warm  sunny 
morning  from  leaves  of  a  moss  ( Funaria )  growing  under  a 
bell-glass  in  the  laboratory.  Figure  a  is  from  a  leaf  taken 
from  a  plant  when  the  dish  was  on  a  table  six  feet  from  the 
nearest  window :  the  chlorophyll  grams  are  against  the 
upper  and  lower  walls  of  the  cells,  presenting  their  maximum 
area  to  intercept  the  light.  This  is  the  typical  daylight  posi- 
tion. Figure  b  is  from  a  leaf  taken  from  a  plant  after  the 


Figure  17.    Cells  from  leaves  of  Funaria  Sp. 
a— cell  from  leaf  in  ordinary  diffuse  daylight, 
b — position  of  chlorophyll-grains  in  intense  light, 
c — position  of  chlorophyll-grains  in  darkness. 

dish  had  been  standing  5  to  10  minutes  in  brilliant  sun- 
shine on  the  window-sill  inside  the  laboratory :  the  chloro- 
phyll grains  are  against  the  lateral  walls  of  the  cells,  pre- 
senting their  minimum  area  to  intercept  the  light.  Figure 
c  is  from  a  leaf  which  had  been  kept  in  the  dark  for  5  to  10 
minutes ;  the  chlorophyll  grains  are  against  the  lateral  and 
bottom  walls  of  the  cells.  These  figures  are  drawn  at  the 
same  magnification,  but  the  cells  and  chromatophores  differ 
considerably  in  size.  Changes  in  position  of  the  chlorophyll 
grains  take  place  very  rapidly,  and  the  drawings  must  be 

*  Frank,  A.  B.  Uber  die  Veranderung  der  Lage  der  Chlorophyllkorner 
und  des  Protoplasmas  in  der  Zelle  und  deren  innere  und  aussere  Ursachen. 
Jahrb.  f.  wiss.  Bot.,  VIII.,  1872.  Stahl,  E.  Uber  den  Einfluss  von  Rich- 
tung  und  Starke  der  Beleuchtung  auf  einige  Bewegungserscheinungen  im 
Pflanzenreiche.  Bot.  Zeitung,  1880.  Oltmanns,  F.  Uber  photometrische 
Bewegungen  der  Pflanzen.  Flora,  Bd.  75,  1892. 


218 


PLANT  PHYSIOLOGY 


made  rapidly  and  at  once,  else  changes  of  considerable  ex- 
tent occur.  Within  two  minutes  after  the  darkened  leaf  is 
put  on  the  microscope,  the  chlorophyll  grains  have  gone 
halfway  to  the  day  position  (a). 

The  behavior  of  the  plate-like  chromatophores  of  the  alga 
Mesocarpus  is  similar.*    When  intense  light  falls  upon   a 


Figure  18.    Cells  of  Mesocarpus  Sp. 
a — cell  with  chromatophore  in  diffuse  daylight  position, 
b — chromatophore  bending  in  too  intense  light, 
c— chromatophore  bent  to  reduce  absorption  of  light. 

Mesocarpus  cell,  the  chromatophore  presents  only  its  edge 
to  the  incident  rays,  but  when  the  light  is  less  intense  the 
chromatophore  bends,  as  in  c,  so  that  the  greater  part 
of  it  presents  its  profile  to  the  light.  Still  less  intense  yet 
strong  light  induces  the  position  indicated  in  b,  and  light 
of  only  moderate  intensity  or  less  is  indicated  by  the  chro- 
matophore being  expanded  at  right  angles  to  the  incident 
rays  as  in  a.  The  chromatophores  change  their  positions 
only  with  a  change  in  the  direction  or  the  intensity  of  the 
illumination,  but  under  favorable  conditions  the  changes 
can  be  accomplished  rapidly. 

Whether  heliotropism  and  heliotaxis  are  or  are  not  funda- 
mentally similar  phenomena  there  is  no  experimental  evi- 
dence for  determining.  The  effect  of  light  upon  the  cell  is 
not  precisely  known,  though  it  is  probable  that  it  chemi- 


Oltmanns    F.    1.  c..  p.  209. 


IRRITABILITY  219 

cally  affects  substances  contained  in  the  cell,  and  that  the 
changes  in  these  stimulate  the  protoplasm  (see  p.  210). 
The  protoplasm  of  one  or  more  cell-members  of  a  tissue  may 
react  by  changing  the  form,  size,  turgor,  or  some  other 
quality  of  the  cells,  thus  causing  a  change  in  the  form  or 
direction  of  an  o^gan  ( heliotropism ) .  This  change  may  be 
made  permanent  by  growth  without  being  directly  accom- 
plished by  growth.  The  same  change  in  form,  size,  turgor, 
or  other  quality  of  isolated  cells  may  result  in  locomotion 
( heliotaxis ) .  In  both  instances  the  effect  of  light  might  be 
intrinsically  the  same,  the  reaction  of  the  protoplasm  to 
chemical  stimulation  also  the  same,  the  result  being  different 
only  because  one  cell  could  move  freely  while  the  other  was 
obliged  to  move  more  than  itself.* 

INFLUENCE  OF  HEAT 

The  degree  of  heat  prevailing  in  the  medium — air,  water, 
soil — in  which  a  plant  is  growing,  and  the  radiant  heat  fall- 
ing upon  the  plant  and  its  immediate  surroundings,  exert 
definite  influences  upon  its  activities.  Without  its  minimum 
degree  of  warmth,  the  plant  will  survive  only  in  a  resting 
condition,  if  at  all.  There  is  also  a  maximum  temperature 
which  can  be  withstood  only  in  the  resting  condition.  Ap- 
parently there  is  no  temperature  so  low  as  to  be  fatal  to 
resting  protoplasm — seeds,  spores,  etc. — and  the  maximum 
for  dry  resting  protoplasm  is  surprisingly  high.  There  are, 
however,  minimum,  optimum,  and  maximum  temperatures 
for  active  protoplasm,  which  are  definite  and  definitely  re- 
lated to  each  other.  Organisms  living  where  the  mean 
temperature  is  low  have  low  minimum  and  maximum  tem- 
peratures. The  native  plants  of  warm  situations  have  cor- 
respondingly high  extreme  temperatures,  and  plants  accus- 
tomed to  living  where  there  is  only  slight  variation  in 
temperature  can  be  cultivated  only  under  similar  con- 

*  Loeb,  J.  Zur  Theorie  der  physiologischen  Licht  und  Schwerkraft- 
wirkungen.  Archiv  f.  d.  gesam.  Physiologic  Bd.  66.  1897.  Einleitung  in 
die  vergleichende  Gehirnphysiologie  und  vergleichende  Psychologic.  Leipzig, 
1899. 


220  PLANT  PHYSIOLOGY 


ditions.     These  statements  are  illustrated  by  the  following 

table  :  * 

min. 

opt. 

max. 

Hydrurus  foetidus 

0° 

10° 

below  16° 

Ulothrix  zonata 

C° 

below  15° 

24° 

Zea  mais 

9° 

34° 

46° 

Cucurbita  pepo 

14° 

34° 

46° 

Bacillus  thermophilus 

42° 

63-70° 

72° 

"       tuberculosis 

30° 

38° 

41° 

It  is  greatly  to  be  regretted  that  those  organisms  which 
apparently  transgress  the  general  laws  expressing  the  rela- 
tions of  active  protoplasm  to  heat  have  been  so  little 
studied  by  physiologists.  The  vegetation  of  geysers  and 
hot-springs,  and  of  the  very  cold  waters  from  glaciers  and 
persistent  snow-deposits,  has  been  studied  too  exclusively  by 
systematists.  The  temperatures  of  the  waters  are  known 
only  in  a  general  way,  the  temperature  of  the  water  actu- 
ally bathing  the  organisms  and  the  exact  condition  of  the 
organisms  have  not  been  accurately  determined.!  Each 
actively  fermenting  manure-heap,  each  mass  of  organic 
matter  so  rapidly  decomposing  that  high  temperatures 
develop  in  it  (e.g.  where  hay  "heats"  in  cock  or  mow),  is 
the  seat  of  bacteria,  the  optimum  temperatures  of  which  are 
above  the  maxima  for  most  forms.  The  difficulty  of  suc- 
cessfully imitating  the  natural  conditions  hampers  experi- 
mental study  of  these  organisms. 

Continental  bodies  of  land  are  subject  to  considerably 
wider  extremes  of  temperature  than  prevail  in  oceanic 
islands  and  in  certain  limited  and  peculiarly  situated  areas 
near  the  coast.  These  show  limited  ranges  of  temperature, 
but  it  is  possible  to  cultivate  there  plants  which  naturally 
occur  much  farther  north  or  south.  This  is  partly  the  rea- 
son why  semi-tropical  and  decidedly  northern  plants  can  be 
successfully  cultivated  in  the  region  about  the  Bay  of  San 

*  See  Pfeffer's  Pflanzenphysiologie,  Bd.  II.,  p.  87,  1901. 

f  Davis,  B.  M.  Vegetation  of  the  hot-springs  of  the  Yellowstone  Park. 
Science.  VI.,  1897.  Tilden,  J.  E.  Observations  on  some  West  American 
thermal  algre.  Bot.  Gaz.,  vol.  25.  1898. 


IRRITABILITY  221 

Francisco,  in  California,  for  there  the  respective  minimum 
and  maximum  temperatures  of  these  forms  are  never  at- 
tained. The  optimum  temperatures  for  most  plants  in  this 
region  are  maintained  for  only  very  short  times,  owing  to 
the  great  daily  range  in  temperature,  but  are  frequently 
repeated,  owing  to  the  slight  seasonal  ranges.  The  frequent 
occurrence  of  optimum  temperatures  may  more  than  com- 
pensate for  their  brief  duration— a  matter,  however,  which 
requires  proof. 

Movements  and  locomotion,  as  well  as  the  rate  of  growth, 
are  more  or  less  controlled  by  heat.  Thermotropism  and 
thermotaxis  are  the  irritable  responses  to  the  stimulus 
of  heat.  A  certain  degree  of  warmth  will  induce  positive 
thermotropism  or  thermotaxis — respectively,  growth  and 
locomotion  towards  the  source  of  heat — while  a  higher  or 
lower  degree  of  heat  may  induce  the  opposite,  negative 
thermotropism  or  thermotaxis.  Roots,  for  instance,  grow 
toward  but  not  into  contact  with  steam  or  hot-water 
pipes  passing  through  the  soil  between  buildings.  Roots  of 
Indian  Corn*  will  grow  toward  the  source  of  heat  till  they 
reach  a  point  where  the  temperature  is  37.5°  C.  They  will 
bend  and  grow  away  from  any  warmer  part  of  the  soil. 
Ordinarily  in  nature,  however,  thermotropic  sensitiveness  is 
of  little  advantage  to  the  plant.  There  is  no  differentiation 
into  sensory  and  motor  zones,  the  whole  growing  region 
being  equally  sensitive.  A  certain  degree  of  warmth  is  re- 
quired in  order  that  the  organism  may  be  sensitive  to  other 
influences.  Thus  heat  must  arouse  the  parasitic  dodder 
( Cuscuta)  to  a  certain  degree  of  sensitiveness  else  it  will  not 
be  able  to  respond  to  the  contact  stimulus  ( see  pp.  244-5 ) 
upon  which  its  attachment  to  the  host  absolutely  depends,  f 

The  sensitiveness  and  the  response  of  motile  organisms  to 
heat  is  manifestly  important.  The  sensitiveness  to  heat 

The  organs  especially  active  in  absorbing  nutrient  aqueous 

*  Wortmann,  J.  Uber  den  Einfluss  der  strahlenden  Warme  auf  wachsende 
Pflanzentheile.  Bot.  Zeitung,  1883.  Tiber  den  Thermotropismus  der  Wurzel. 
Bot.  Zeitung,  1885. 

fPeirce,  G.  J.  A  contribution  to  the  physiology  of  the  Genus  Cuscuta. 
Annals  of  Bot..  Vol.  VIII.,  1894. 


222  PLANT  PHYSIOLOGY 

which  causes  a  man  who  is  cold  to  approach  the  fire  but 
not  to  draw  too  close,  and  when  wrarmed  to  move  to  a 
slightly  cooler  spot,  is  not  peculiar  to  him  or  to  higher 
animals.  This  example  of  thermotaxis  expresses  the  sensi- 
tiveness of  all  living  protoplasm  and  the  habit  of  all  motile 
organisms  to  seek  positions  which  are  most  comfortable 
because  most  favorable  to  the  accomplishment  of  their  vital 
functions.  Thus  slime-moulds*  and  zoosporest  are  known 
to  move  toward  sources  of  heat. 

Whether,  corresponding  with  the  influence  of  light,  the 
direction  of  movement  of  the  internal  organs  of  the  cell  is 
affected  by  the  direction  of  the  source  of  heat  seems  not 
to  have  been  studied.  The  rate  of  movement  of  the 
organs  of  the  cell  in  relation  to  the  degree  of  warmth  has 
been  carefully  examined.  For  example,  the  chlorophyll 
grains  in  Elodea,  Valisnerhi,  and  Chara  circulate  with  in- 
creasing rapidity  up  to  a  temperature  of  34  to  38°  C.,  but  at 
44°  all  movement  ceases,  though  it  will  be  resumed  when 
the  temperature  is  lowered.]:  The  temperature  at  which  the 
protoplasm  circulates  most  rapidly  in  these  plants  is  near 
the  optimum  for  most  of  the  activities  of  the  cell. 

INFLUENCE  OF  WATER 

A  sufficient  quantity  of  water  is  a  necessary  condition 
for  active  life  (p.  6 ) .  Water  is  also  an  essential  component 
of  the  living  structure,  protoplasm  (p.  7).  Given  the 
minimum  amount  of  water  for  the  erection  of  the  living 
structure  after  a  period  of  inaction,  the  organism  will  re- 
sume its  functions.  The  germination  of  seeds  and  spores 
shows  this.  Increase  in  the  amount  of  water  from  the 
minimum  to  the  optimum  is  followed  by  increase  in  all  the 
vital  activities.  This  is  consistent  with  chemical  and  physi- 
cal experience  in  the  laboratory,  a  minimum  quantity  of 
water  being  indispensable  to  many  reactions,  a  larger  (  opti- 

*  Stahl,  E.    Zur  Biologic  der  Myxomyceten.  Bot.  Zeitung.  1884. 

f  Strasburger,  E.  Wirkung  des  Lichtes  und  der  Warme  auf  Schwarm- 
sporen.  Jenaischer  Zeitschrift,  1878. 

%  Velten.  W.  Einwirkung  der  Temperatur  auf  die  Protoplasmabewegung. 
Flora,  1876. 


IRRITABILITY  223 

mum )  amount  causing  them  to  take  place  more  rapidly  or 
more  energetically.  There  are,  therefore,  minimal  and  opti- 
mal quantities  for  the  ptrvsical  states  and  the  chemical 
processes  in  the  living  cell  and  the  living  organism,  but 
until  water  is  sufficient  in  amount  to  become  the  vehicle  of 
mechanical  forces,  there  cannot  be  said  to  be  any  maximum 
quantity.  The  minimum  and  optimum  vary  with  the  or- 
ganism. 

An  organism  will  absorb  water  up  to  a  given  amount, 
but  it  can  absorb  no  more  (see  p.  110)  because  the  sub- 
stances which  it  contains  do  not  offer  the  physical  condi- 
tions making  further  absorption  possible.  The  absorption 
of  water  gives  rise  to  changes  in  the  physical  conditions  pre- 
vailing within  the  cells  ( pp.  210-11 ) ,  the  changes  manifesting 
themselves  most  conspicuously  in  an  increase  in  volume  or 
pressure,  or  both.  These  physical  changes  directly  affect  the 
living  protoplasm.  The  greater  pressure  or  larger  volume 
of  individual  cells  enables  them  to  exert  greater  mechanical 
force  upon  their  surroundings  and  to  occupy  more  space ;  it 
enables  the  organ  which  they  compose  to  expand  or  to  come 
into  a  more  favorable  position,  etc.  Besides  these  mechani- 
cal effects  of  water,  which  influence  the  living  protoplasm, 
the  greater  or  less  quantity  of  water  in  the  cell  exercises  a 
direct  physiological  effect  upon  the  activities  of  the  cell. 
When  the  plant  or  the  cell  needs  water,  its  activities  will  be 
reduced,  within  certain  limits,  in  proportion.  Conversely, 
any  increase,  under  these  conditions,  hi  the  amount  of  water 
available  will  stimulate  the  plant  or  the  cell. 

Water  exerts  two  distinct  influences  upon  the  plant.  This 
can  best  be  shown  by  examining  the  directive  influence  of 
water  upon  growth.  First,  let  the  downward-growing  pri- 
mary root  of  a  seedling  growing  in  air  not  excessively  damp 
be  exactly  measured  as  to  diameter  and  length  by  the  eye- 
piece micrometer  of  a  horizontal  microscope;  then  plunge 
this  root  into  water  of  the  same  temperature  as  the  air  and 
note  the  increase  in  dimensions  which  promptly  follows. 
This  is  a  purely  mechanical  effect,  swelling.  Again,  let  seed- 
lings be  grown  in  any  apparatus  which  will  expose  the 
roots  to  unequal  amounts  of  moisture  on  opposite  sides ;  for 


224  PLANT  PHYSIOLOGY 


instance,  Sachs's  familiar  cylinder  of  wire-netting  filled  with 
damp  moss  or  moist  earth.*  If  this  cylinder  be  hung 
slanting  at  an  angle  of  about  45°,  the  roots  will  not  grow 
vertically  downward  away  from  the  cylinder,  but  will  follow 
along  its  under  side  in  spite  of  the  geotropic  stimulus  (p. 
204).  In  this  experiment  the  roots  bend  in  the  direction 
from  which  they  obtain  water.  In  the  first  experiment  the 
absorption  of  water  is  followed  by  expansion,  swelling. 
In  the  second  experiment  that  side  expands  which  is  not 
able  directly  to  absorb  water,  while  that  side  which  does 
absorb  water  does  not  expand  as  much.  In  the  first 
experiment  the  physical  influence  is  uniform  on  all  sides 
of  the  root,  which  swells  uniformly.  In  the  second  ex- 
periment, the  physical  influence  was  greater  on  one  side 
than  on  the  other,  the  tendency  to  swell  on  one  side 
was  offset  by  the  increased  activity  of  the  protoplasm, 
the  more  rapid  growth,  of  the  opposite  side,  in  response  to 
stimulus.  Such  a  reaction  as  this  is  called  hydrotropism. 

The  hyphse  of  Mucor  grown  on  moist  bread  over  water 
are  deflected  when  they  reach  the  surface  of  the  water,  grow- 
ing instead  horizontally  over  it  and  against  the  bread. 
This  bending  away  from  water  is  not  negative  hydrotro- 
pism, but  rather  the  reaction  of  the  hyphae  to  other  stim- 
uli such  as  food,  oxygen,  etc.,  which  are  more  abundant  in 
the  bread  than  in  the  water. 

Sachs's  experiment  (see  above),  with  roots  deflected  by 
moisture  from  the  vertical  position,  reveals  a  very  important 
principle.  It  is  manifestly  more  important  for  the  root  as 
an  absorbing  organ  to  grow  where  it  will  obtain  needed 
aqueous  solutions  than  for  it  to  be  in  any  particular  posi- 
tion with  relation  to  the  vertical;  but  in  order  that  the 
older  parts  of  the  root  may  give  the  needed  mechanical 
support  to  the  plant,  the  root  must  grow  downward  into 
the  soil.  There  must  therefore  be  a  delicate  balance  in  the 
sensitiveness  and  the  reactions  to  geotropic  and  hydro- 
tropic  stimuli. 

*  Sachs,  J.  von.  Uber  die  Ablenkung  der  Wurzeln  von  ihrer  normalen 
Wachsthumsrichtimg  durch  feuchte  Korper  (Hydrotropismus).  Arb.  d. 
bot.  Inst.  Wiirzburg,  Bd.  I.,  1871-4.  Gesam.  Abhandl.,  Bd.  II.,  1893. 


IRRITABILITY  225 

solutions — roots,  rhizoids,  *  pollen-tubes,  f  etc. — are  the  ones 
most  sensitive  to  differences  in  the  proportions  of  water  in 
their  environment.  Other  organs  are  relatively  or  quite 
indifferent.  Most  stems  are  not  hydrotropically  sensitive, 
but  the  peculiar  stem  of  the  parasite,  dodder  (Cuscuta), 
which  must  twine  closely  about  living  plants  and  extract 
nourishment  from  them,  is  somewhat  sensitive.! 

The  size  and  number  of  root-hairs  is  inversely  propor- 
tional to  the  ease  with  which  the  plant  obtains  the  water  it 
needs.  The  difficulty  of  absorbing  water  in  sufficient  quan- 
tity to  meet  the  demand  made  by  transpiration  and  evapo- 
ration ( see  p.  141 )  from  the  aerial  parts  acts  as  the  stim- 
ulus to  the  young  epidermal  cells  of  the  root  to  grow  larger  and 
into  such  form  that  they  will  be  able  more  readily  to  absorb 
the  water  held  on  the  surfaces  of  the  soil  particles.  An 
amount  of  water  in  the  soil  less  than  the  optimum  is  fol- 
lowed by  elongation  of  the  epidermal  cells,  and  they  grow 
out  as  hairs.  Seeds  germinated  in  moist  air  instead  of  in 
damp  soil  develop  a  larger  number  of  still  longer  hairs.  On 
the  other  hand,  the  roots  of  seedlings  growing  in  water  are 
nearly  or  quite  bare  of  hairs.  The  stimulus  to  the  produc- 
tion of  root-hairs  may  develop  within  the  plant  itself.  It 
may  consist  in  the  osmotic  withdrawal  of  water  from  the 
epidermal  cells  to  make  good  the  losses  above.  In  any  case, 
water  exerts  the  stimulus  to  growth,  and  the  living  proto- 
plasm reacts  to  the  stimulus. 

The  root-hairs  grow  toward  and  around  soil  particles. 
These  hold  films  of  water  on  the  surface.  The  growth  of  the 
root-hairs  toward  the  soil  particles  is  evidently  positively 
hydrotropic.  The  growth  of  the  hairs  around  the  particles 
may  be  the  combined  result  of  the  stimulus  of  contact  with 
the  solid  (see  p.  241)  and  of  water. 

The  opposite  result,  the  growth  of  parts  away  from  water, 

*  Molisch,  H.  Untersuchungen  iiber  den  Hydrotropismus.  Sitzb.  d.  K.  K. 
Akad.  der  Wissv  Wien,  1884. 

f  Miyoshi,  M.  Uber  Reizbewegung  der  Pollenschlauche.  Flora,  Bd.  78, 
1894. 

J  Peirce,  G.  J.  Contribution  to  the  physiology  of  the  Genus  Cuscuta. 
Annals  of  Botany,  vol.  VIII. .  1894. 


226  PLANT  PHYSIOLOGY 

is  shown  by  the  sporangiophores  of  some  of  the  fungi.  It 
may  well  be  that  the  fruiting  organs  of  other  plants  depen- 
dent upon  dry  air  for  the  best  dispersal  of  their  seeds  or 
spores  are  also  negatively  hydrotropic.  * 

Hydrotaxis  is  exhibited  by  many  of  the  slime-moulds,  for 
example  .Ethalium  (Fulligo).  Stahlt  shows  that  the  dis- 
tribution of  these  plants  on  and  in  the  rotting  wood,  etc. 
where  they  live,  corresponds  to  the  distribution  of  water 
there;  they  are  positively  hydrotactic. 

The  locomotion  of  such  plants  as  are  motile  at  any  time 
is  dependent  upon  water,  for  this  is  the  only  medium 
through  which  they  can  propel  themselves.  Many  other 
movements  than  those  of  locomotion  are  also  dependent 
upon  water;  the  withdrawal  of  water,  or  variation  in  the 
amount  of  water,  may  cause  purely  physical  changes  in  cell- 
walls  or  dead  tissues  which  result  in  the  exercise  of  me- 
chanical force.  These  hygroscopic,  movements,  such  as  the 
opening  of  the  peristome  in  mosses,  the  action  of  the  elaters 
of  liverworts  and  Equisetum,  the  opening  of  fern-sporangia, 
.the  curling  of  the  awns  of  Geraniaceae,  the  splitting  open 
of  pods,  etc.,  etc.,  are  free  from  the  action  of  living  proto- 
plasm, and  are  mentioned  in  connection  with  irritable  phe- 
nomena only  to  make  the  difference  clear. 

INFLUENCE  OF  OTHER  SUBSTANCES  THAN  WATER 

In  the  whole  range  of  chemical  substances  there  are  ex- 
tremely few  which  never  in  any  way  affect  living  protoplasm. 
Many  substances,  however,  affect  protoplasm  only  mechani- 
cally— as  obstacles  in  the  way  of  its  growth  or  movement, 
or,  like  the  nitrogen  of  the  air,  which  blows  against  and 
subjects  it  to  strain,  etc.  (For  the  effects  of  these  see  p. 
189).  Unless  a  substance  is  soluble  in  water  it  can  af- 
fect protoplasm  in  no  other  than  mechanical  ways.  Of  the 
soluble  substances  some  affect  protoplasm  favorably,  others 
unfavorably,  others  not  at  all.  When  an  effect  is  produced, 

*  Preliminary  experiments  on  Fimhriaria  California,  lead  me  to  believe 
that  the  sporophyte  of  this  and  of  other  liverworts  (e.  g.  Anthoceros) 
may  be  negatively  hydrotropic. 

f  Stahl,  E.     Zur  Biologic  der  Myxomyceten.    Bot.  Zeitung,  1884. 


IRRITABILITY  227 

it  may  be  purely  physical  or  it  may  be  chemical,  involving 
some  change  in  composition.  The  same  substance  in  differ- 
ent concentrations,  may  produce  favorable  or  unfavorable 
effects  or  apparently  none,  and  the  effects  may  be  first 
physical  and  then  chemical,  or  vice  vei'sa.  The  effect  pro- 
duced is  dependent  upon,  1st,  the  acting  substance,  2d,  the 
organism,  3d,  the  conditions  prevailing  at  the  time.  The 
organism  will  vary  at  different  times  in  its  activities,  con- 
tents, and  composition,  and  will  therefore  vary  in  its  reac- 
tion or  response  to  external  influences. 

In  studying  the  influence  of  gravity  upon  plants,  wre  were 
dealing  with  a  constant  force  and  a  varying  organism.  In 
studying  the  influence  of  light,  we  were  dealing  with  a  vary- 
ing force  as  well  as  var}ring  organism,  but  the  force  pro- 
duces its  effects  always  by  the  same  means.  In  studying  the 
influence  of  water,  we  were  dealing  with  a  single,  simple 
compound  varying  in  quantity,  but  in  its  effects  only 
because  of  this  fact  and  of  the  varying  organism.  In  study- 
ing the  influence  of  other  substances  upon  protoplasm 
we  have  to  deal  with  their  relations  to  water  and  to  each 
other  as  well  as  to  the  organism.  This  materially  compli- 
cates the  subject.  The  effects  of  common  salt  illustrate 
these  several  points.  A  very  dilute  solution  of  common  salt 
(Nad)  acts  unfavorably  upon  plants  the  roots  of  which 
are  bathed  in  it.  The  sodium  and  chlorine  atoms  become 
dissociated  and  act  as  independent  poisons.  Whether  the 
poisonous  action  of  these  atoms  is  due  to  their  own  chemi- 
cal properties  or  to  their  electrical  charges  is  not  certain 
and  at  all  events  need  not  now  be  discussed.*  A  solution  of 
such  concentration  that  the  atoms  are  not  dissociated  and 
that  molecules  remain  intact  will  have  no  effect.  Still  fur- 
ther concentration  will  cause  plasmolysis  of  the  living  cells. 

*  Kahlenberg.  L..,  and  True,  R.  H.  On  the  toxic  action  of  dissolved 
salts  and  their  electrolytic  dissociation.  Bot.  Gazette,  XXII.,  1896.  True, 
R.  H.  The  physiological  action  of  certain  plasmolyzing  agents.  Bot.  Ga- 
zette, XXVI.,  1898.  Heald.  F.  D.  On  the  toxic  effect  of  dilute  solutions 
of  acids  and  salts  upon  plants.  Bot.  Gazette,  XXII.,  1896.  Loeb.  J.  On 
ion-proteid  compounds  and  their  role  in  the  mechanics  of  life-phenomena. 
I.  The  poisonous  character  of  a  pure  NaCl  solution.  Amer.  Journ. 
Physiol.,  vol.  III..  1899-1900. 


228  PLANT  PHYSIOLOGY 

This  in  itself  is  injurious,  though  the  cells  may  recover  their 
turgor,  becoming  adjusted  to  the  greater  density.  Plants 
continually  exposed  in  nature  to  salt  solutions  of  considera- 
ble density  present  a  very  different  appearance  from  those 
growing  under  more  usual  conditions.*  They  are  usually 
fleshier  and  coarser,  often  more  compact  in  habit,  than  ordi- 
nary land  plants  ( compare  Solidago  sempervirens  with  the 
other  Solidagos  of  New  England,  the  marsh  grasses  with 
those  of  meadows,  etc.). 

The  theory  of  electrolytic  dissociation  of  the  atoms  of 
substances  dissolved  in  proportionally  large  volumes  of ' 
water  is  extremely  useful  to  the  physicist  and  chemist.  It 
has  been  profitably  employed  by  some  physiologists  in  dis- 
cussing the  poisonous  effects  of  certain  substances  ordinarily 
harmless.  The  theory  is  useful,  but  it  is  very  far  from  ex- 
perimental proof.  Nevertheless  it  may  be  employed  in  plant 
physiology  to  suggest  possible  explanations  of  otherwise 
incomprehensible  phenomena.  We  may  conceive  the  cell-sap 
— an  aqueous  solution  of  a  great  number  of  substances  in  very 
various  and  inconstant  amounts — as  containing  not  only 
substances  still  in  the  molecular  state,  but  also  substances 
in  a  state  of  atomic  dissociation,  in  which  the  component 
atoms  are  scattered  at  considerable  distances  from  each 
other  throughout  the  solution.  Whether  these  dissociated 
atoms  or  groups  of  atoms  smaller  than  molecules  are 
charged  with  electricity — some  positive,  others  negative — or 
whether  the  atoms  possess  only  their  chemical  properties 
and  affinities  is  by  no  means  known.  Certain  it  is,  how- 
ever, if  there  are  dissociated  atoms  in  the  cell-sap,  that 
these  atoms  are  more  susceptible  to  physical  and  chemical 
influences  within  and  without  the  cell,  and  more  prompt 
and  active  in  response,  than  are  atoms  associated  in  mole- 
cules. Dissociated  atoms  exhibit  the  properties  of  the 
chemical  elements,  whereas  associated  atoms  present  only 
the  properties  of  compounds.  The  elements  are  more  ready 
for  combination  than  compounds  are  for  recombination.  A 
dilute  solution,  one  containing  dissociated  atoms,  is  there- 

*  Schimper,  A.  F.  W.    Pflanzengeographie   auf  physiologiscker  Grund- 
lage.    Jena,  1898. 


IRRITABILITY  229 

fore  more  sensitive  to  external  influences  and  is  more  likely 
to  change  than  is  a  concentrated  solution.  Some  of  the 
substances  in  the  cell-sap  may  be  present  in  such  minute 
quantities  as  to  be  dissociated.  In  this  case  these  dissoci- 
ated atoms,  and  in  consequence  the  cell-sap  as  a  whole, 
would  be  very  sensitive  to  external  influences,  physical  and 
chemical.  Anything  influencing  the  cell-sap  in  any  way 
immediately  affects  the  protoplasm  through  the  cell-sap 
which  permeates  every  part  of  it.  This  influence  upon  the 
protoplasm  may  be  chemical  or  it  may  be  electrical.  Only 
the  future  can  decide  which  it  is,  but  meantime  this  concep- 
tion assists  us  in  appreciating  how  the  protoplasm  may  be 
affected  and  how  the  stimulus  may  be  promptly  transmitted 
from  cell  to  cell.* 

Any  substance  or  any  force  which  disturbs  the  balance 
between  the  constructive  and  destructive  processes  main- 
tained by  the  organism  or  the  cell  will  affect  the  organism, 
producing  an  irritation  equal  to  the  disturbance.  If  the  new 
balance  necessitated  by  the  introduction  of  the  new  sub- 
stance or  the  new  force  is  more  or  less  favorable  to  the 
accomplishment  of  the  vital  processes  than  the  old,  the  liv- 
ing cell  or  organism  will  respond  accordingly.  Organisms 
living  in  certain  situations  are  exposed  to  very  frequent  and 
very  rapid  changes  in  their  environment.  Thus  the  plants 
living  between  the  high  and  low  tide  marks  on  the  sea- 
coast  are  subjected  to  a  twice  daily  change  in  their  rela- 
tions to  water.  If  rain  falls  upon  them  while  they  are 
bared  by  the  tide,  they  are  subjected  to  still  another  change. 
Marine  plants  living  at  the  mouths  of  fresh-water  streams 
are  also  exposed  to  frequently  repeated  and  very  rapid 
changes  in  the  composition  and  density  of  the  medium  which 
surrounds  them.  Such  changes  in  osmotic  influences  imply 
a  very  great  range  of  adaptability  on  the  part  of  the 
plants  regularly  exposed  to  them.  Plants  living  in  situa- 
tions where  osmotic  influences  change  only  gradually  or 
slightly,  as  in  pools  which  dry  up  during  the  summer,  or  in 
bodies  of  water  which  are  added  to  only  occasionally  or 

*  Matthews,  A.  P.  The  nature  of  the  nerve  impulse.  Century  Magazine, 
vol.  63,  1902.  Abstract  also  in  Science,  vol.  15,  p.  344.  1902. 


230  PLANT  PHYSIOLOGY 

seasonally,  are  far  more  susceptible  to  changes  in  the  den- 
sity of  the  water.  It  has  been  suggested  that  the  recourse 
of  fresh  water  algae  to  their  various  modes  of  reproduction 
(see  pp.  268-9)  may  be  a  reaction  to  osmotic  differences 
in  the  water  in  which  they  live,*  but  this  is  by  no  means 
assured. 

Certain  substances,  nutritious  and  other,  stimulate  the 
growth  and  other  activities  of  plants  as  of  animals.  Small 
amounts  of  poisonous  matters,  such  as  ether,  t  metallic 
salts,  cocaine,  and  morphine]:  act  as  stimulants.  The  prin- 
ciple governing  the  action  of  these  substances  is  the  same 
as  that  controlling  the  use  of  tea,  coffee,  and  tobacco  by 
human  beings,  for  all  of  these  drugs  are  dangerously  de- 
pressing poisons  when  taken  in  concentrated  form.  Alcohol 
may,  in  both  animals  and  plants,  serve  as  a  source  of 
energy  by  oxidation  and  thereby  stimulate  the  organism.  § 

One  word  must  be  said  about  the  startling  claim  that 
definite  chemical  stimuli  may  replace  the  male  elements  in 
sexual  reproduction.  Loeb  §§  reports  that  he  has  reared  sea- 
urchins  through  the  earlier  larval  stages  from  unfertilized 
eggs  by  subjecting  the  fggs  to  suitable  chemical  stimuli. 
From  this  he  argues  that  the  main  function  of  the  sperm 
is  to  supply  the  eggs  with  certain  compounds  indispensable 
to  further  development,  and  that  when,  no  matter  by  what 

*  Livingston,  B.  E.  Nature  of  the  stimulus  which  causes  change  of 
form  in  polymorphic  algae,  Bot.  Gazette,  vol.  30,  1900.  Further  notes 
on  the  physiology  of  polymorphism  in  green  algae.  Bot.  Gazette,  vol.  32, 
1901. 

t  Townsend,  C.  O.  The  correlation  of  growth  under  the  influence  of 
injuries.  Annals  of  Bot.,  XI..  1897. 

t  Richards,  H.  M.  Die  Beeinflussung  des  Wachsthums  einiger  Pilze  durch 
chemische  Reize.  Jahrb.  f.  wiss.  Bot.,  Bd.  30,  1897.  See  also  Raciborski, 
M.  Uber  den  Einfluss  ausserer  Bedingungen  auf  die  Wachsthumsweise  des 
Basidiobolus  ranarum.  Flora,  Bd.  83,  1896. 

§  Atwater.  W.  O.  Metabolism  of  matter  and  energy  in  the  human  body. 
Bull.  U.  S.  Dept.  Agriculture,  Washington,  1901. 

§§  Loeb,  J.  On  the  nature  of  the  process  of  fertilization  and  the  artificial 
production  of  normal  larvae  (plutei)  from  the  unfertilized  eggs  of  the 
sea-urchin.  On  the  artificial  production  of  normal  larvae  from  the  un- 
fertilized eggs  of  the  sea-urchin  (Arbacia) ,  Amer.  Journ.  Physiol.,  vol.  III., 
1899-1900. 


IRRITABILITY  231 

means,  these  compounds  are  supplied,  development  will  go 
on.,  This  may  be  true,  but  that  it  is  true  is  far  from  proved. 
There  are  well-known  cases  of  parthenogenesis  among  both 
animals  and  plants.  From  these  one  might  infer  either  that 
no  eggs  require  fertilization  in  order  to  develop,  or  that 
these  eggs  are  chemically  stimulated  to  develop  by  the  sub- 
stances in  air  or  water.  Both  of  these  inferences  would  be 
unjustified.  Sea-urchin  eggs  may  not  develop  partheno- 
genetically  when  let  alone,  though  they  may  do  so  when 
chemically  stimulated.  But  it  is  a  far  cry  from  Loeb's 
brilliant  experiments  to  the  generalization  that  fertilization 
is  principally  chemical  stimulation.  In  many  cases,  the 
structure  of  the  egg  must  be  completed  by  the  material  of 
the  sperm  before  normal  development  can  begin.  This,  in  a 
sense,  is  chemical  stimulation,  but  it  is  also  a  morphological 
process.  Fertilization,  in  most  cases  at  least,  consists  in  the 
completion  of  the  structure  of  the  egg  as  weU  as  in  the 
stimulation  of  its  living  protoplasm  to  grow,  etc. 

The  direction  of  growth,  as  well  as  the  rate  and  kind,  is 
influenced  by  other  substances  than  water.  This  response  of 
the  living  plant  to  external  stimuli  is  called  chemotropism. 
It  is  well  known  that  roots  grow  toward,  around,  and  into 
especially  fertile  clods  of  earth,  leaving  the  less  nutritious. 
The  distribution  of  oxygen,  carbon-dioxide,  and  possibly  of 
other  gases  in  the  soil  (e.  g.  illuminating  gas  leaking  from 
underground  pipes)  affects  the  growth  of  the  roots.  *  They 
will  not  grow  far  down  into  water ;  they  will  turn  up  when- 
ever their  supply  of  air  faUs  below  the  optimum.  They  will 
grow  toward  the  sources  of  many  gases,  but  so  soon  as 
they  reach  a  point  where  the  amount  of  gas  becomes  at  all 
considerable,  they  will  bend  and  grow  away. 

The  chemotropic  sensitiveness  of  hyphaef  and  of  pollen- 
tubes  J  is  marked  and  important.  Miyoshi  finds  that  the 

*  Molisch.  H.  Fber  die  Ablenkung  der  Wurzeln  von  ihrer  norraalen 
Wachsthumsrichtung  durch  Case  (Aerotropismus) .  Sitzb.  d.  Akad.  d.  Wiss.. 
Wien.  Bd.  91.  I..  1884. 

4  Miyoshi.  M.     Uber  Chemotropismus  der  Pilze.    Bot.  Zeitung    1894. 

i  Miyoshi,  M.  Uber  Reizbewegungen  der  Pollenschlanche.  Flora.  Bd. 
78.  1894.  Lidforss,  B.  fl)er  den  Chemotropismus  der  Pollenschlauche. 
Ber.  d.  Deutsch.  Bot.  Ges.  Bd.  17.  1899. 


232  PLANT  PHYSIOLOGY 


spores  of  various  fungi,  germinated  on  finely  perforated  films 
of  collodion  or  of  stomata-bearing  epidermis  floating  on 
water  or  solutions  of  various  substances,  or  on  the  stomata- 
bearing  surface  of  Tradescantia  leaves  injected  under  the  air- 
pump  with  solutions  of  the  substances  to  be  tested,  send 
out  hyphae  which  grow  toward  and  through  the  perforations 
into  the  solution,  or  away  from  the  perforations,  according 
as  the  substances  used  are  attractive  or  repellent.  The 
attractive  substances  are  not  all  nutritious,  the  repellent 
not  all  poisonous.  Osmotic  and  dissociation  phenomena 
may  be  concerned.  Glycerine  and  gum  arabic,  nutritious 
though  they  are,  do  not  attract.  Miyoshi  and  Lid- 
forss  report  that  the  direction  of  growth  of  pollen-tubes  is 
influenced  by  water,  oxygen,  sugars,  and  easily  diffusible 
proteids,  especially  those  composing  and  associated  with 
enzyms.  All  of  these  are  present  in  various  amounts  in  the 
different  parts  of  stigma,  style,  and  ovary.  Lidforss  says 
that  the  direct  growth  of  the  pollen-tube  toward  the  micro- 
pyle  is  due  solely  to  hunger — certainly  a  graphic  expression. 
On  scattering  various  kinds  of  pollen  and  the  spores  of 
Mucor  with  ovules  of  Scilla  on  moist  agar-agar,  Miyoshi 
found  Mucor  hyphse  and  pollen-tubes  growing  toward  and 
even  into  the  ovules. 

The  meeting  of  the  conjugation  tubes  of  Spirogyra  cells 
could  hardly  occur  with  such  regularity  if  it  were  merely  a 
matter  of  chance.  On  the  other  hand,  experimental  proof 
that  their  growth  toward  one  another  is  chemotropically 
directed  is  so  difficult  that  no  attempts  have  yet  succeeded. 
It  has  been  noticed*  that  bacteria  are  attracted  to  those 
portions  of  Spirogyra  cells  which  grow  out  into  tubes. 
From  this  it  is  inferred  that  some  special  substance  diffuses 
from  the  tubes.  This  substance  may  not  induce  the  cell  in 
an  adjacent  filament  to  put  out  a  tube,  yet  it  may  so  di- 
rect the  growth  of  a  tube  already  forming  that  the  two  will 
meet.  Too  much  reliance  should  not  be  placed  on  inference 
from  such  evidence,  however,  for  though  the  behavior  of 

*  Overton,  C.  E.  tiber  den  Conjugationgvorgang  bei  Spirogyra.  Ber.  d. 
Deutsch.  Bot.  Ges.,  VI.,  1888.  Haberlandt,  G.  Zur  Kenntniss  der  Conju- 
gation bei  Spirogyra.  Sitzb.  d.  K.  Akad.  d.  Wigs.,  Wien,  Bd.  99, 1.  1890. 


IRRITABILITY  233 

bacteria  where  only  one  kind  is  concerned  may  be  definitely 
known,  the  behavior  of  a  swarm  of  several  or  many  kinds 
is  due  partly  to  the  influence  of  the  bacteria  themselves 
upon  each  other. 

One  of  the  most  striking  responses  to  chemical  irritation 
is  that  exhibited  by  the  tentacles  on  the  leaves  of  the  vari- 
ous species  of  the  carnivorous  Sundew,  Drosera.  The 
thorough  investigation  of  this  by  Darwin*  has  placed  our 
knowledge  on  a  very  definite  basis.  He  found  that  the  ten- 
tacles would  bend  over  and  the  leaves  curl  with  a  prompt- 
ness and  to  a  degree  proportional  to  the  irritating  quality 
of  the  substance  used.  That  it  was  not  mere  contact  that 
caused  the  irritation  was  experimentally  demonstrated  by 
placing  bits  of  wood,  quartz,  etc.,  and  drops  of  milk,  beef 
infusion,  etc.,  upon  the  tentacles  of  expanded  leaves.  The 
wood,  quartz,  etc.,  did  not  induce  the  tentacles  to  close  over 
them ;  the  drops  of  nutrient  liquid  did.  Darwin  then  tested 
the  influence  of  a  great  variety  of  substances  and  found 
that  some  did,  others  did  not,  stimulate  the  tentacles  to 
close  over  them. 

The  effects  of  fragrant  substances  in  directing  the  locomo- 
tion of  animals  is  a  fact  of  such  universal  experience  that 
we  may  well  use  it  as  the  starting-point  of  our  discussion  of 
chemotaxis.  Flies  are  attracted  by  the  odors  of  cooking 
and  repelled,  as  every  camper  knows,  by  a  smoky  fire. 
Walking,  crawling,  and  swimming  animals  are  attracted  or 
repelled  by  soluble  substances,  either  in  the  solid  form  or  in 
solution.  Ants  will  follow  a  trail  of  grains  of  sugar  to  its 
source,  but  they  will  be  turned  from  their  path  by  un- 
pleasant substances  placed  in  their  way.f  Insects,  and 
doubtless  birds  also,  concerned  in  the  cross-pollination  of 
flowers,  are  attracted  from  a  distance  by  the  odorous  sub- 
stances formed  in  them.  Only  when  the  animal  comes  near 
enough  for  the  flower  to  be  within  its  range  of  vision 

*  Darwin,  C.  Insectivorous  Plants.  On  cytological  changes  in  gland 
cells  following  stimulation  see  Huie,  L.  H.,  in  Quart.  Journ.  Microsc.  Sci.. 
vols.  39  and  42?  1896  and  1899;  and  Rosenberg.  O..  Physiolog.-cyto- 
log.  Untersuch.  iiber  Drosera  rotundifolia.  Upsala,  1899. 

f  Lubbock,  Sir  John.    Ants.  Bees,  and  Wasps.    New  York,  1884. 


234  PLANT  PHYSIOLOGY 

is  it  attracted  by  the  distinguishing  color  and  form.  The 
direction  of  locomotion  by  taste  and  scent  is  but  the  special 
form  of  chemotaxis  exhibited  by  higher  animals.  In  the 
lower  animals  and  in  plants,  in  which  the  separate  senses 
are  not  seated  in  specially  differentiated  organs,  the  same 
sensitiveness  of  the  living  protoplasm  is  exhibited,  in  a  de- 
gree not  always  less  striking. 

The  habit  of  motile  aerobic  bacteria  of  gathering  around 
air-bubbles  and  at  the  edges  of  the  coverglass  in  micro- 
scopic preparations  is  of  common  observation.  Correspond- 
ingly, in  preparations  made  without  air-bubbles  and  cut  off 
from  a  supply  of  air  by  sealing  the  margins  with  vaseline, 
the  bacteria  become  uniformly  distributed  and  presently 
come  to  rest.  This  indicates  the  dependence  of  such  bac- 
teria upon  oxygen  as  a  source  of  energy.  Their  uniform 
distribution  when  the  supply  of  oxygen  is  uniform,  and 
their  collecting  around  the  greater  quantities  when  the 
supply  is  scattered  irregularly,  indicate  the  importance  of 
oxygen  in  directing  as  well  as  maintaining  their  movements. 
The  chemotactic  movement  of  certain  bacteria  in  relation  to 
oxygen  was  carefully  studied  by  Engelmann*  and  made  use 
of  by  him  in  determining  which  of  the  component  rays  of 
sunlight  are  most  efficient  in  the  manufacture  of  food  by  the 
chlorophyll  grain  (see  p.  56).  By  placing  a  filamentous 
alga  under  the  coverglass  with  the  bacteria,  and  duly  illu- 
minating the  preparation,  the  alga  will  become  photosyn- 
thetically  active  and  liberate  oxygen,  the  bacteria  gather- 
ing about  it.  If  a  short  spectrum,  instead  of  white  light,  be 
thrown  on  the  alga,  the  cells  exposed  to  the  yellow  light 
will  give  out  most  oxygen,  as  shown  by  the  largest  num- 
ber of  bacteria  collecting  there. 

The  chemotactic  movements  of  bacteria  with  relation  to 
conjugating  Spirogyra  filaments  have  already  been  referred 
to  (p.  232).  Bacteria  are  known  to  collect  on  the  surface 

'*  Engelmann,  T.  W.  Eine  neue  Methode  zur  Untersuchung  der  Sauer- 
stoffausscheidung  pflanzlicher  und  thierischer  Organismen.  Archiv.  f.  d. 
ges.  Physiol.,  Bd.  25,  1881.  L'emission  d'oxygene  sous  Tinfluence  de  la 
lumiere  par  les  cellules  a  chromophylle,  demonstres  au  moyen  de  la 
methode  bacterienne.  Archives  Neerland.,  T.  28.  1894. 


IRRITABILITY  235 

of  decaying  matters,  attracted  thither  by  the  substances 
diffusing  from  the  dead  organic  material.  The  sensitiveness 
of  the  bacteria  of  disease  to  the  dissolved  substances  in  the 
body  of  their  host  enables  them  to  move  toward  and  into 
the  places  most  favorable  to  them.  The  chemotactic  sensi- 
tiveness of  bacteria,  flagellates,  Volvocinste,  and  motile 
reproductive  bodies  was  studied  very  thoroughly  by  Pfeffer.  * 
His  method  consisted  in  the  employment  of  capillary  tubes 
closed  at  one  end  and  partly  filled  under  the  air-pump 
with  solutions  of  the  substances  to  be  tested.  These  tubes, 
after  rinsing,  he  introduced  into  drop-cultures  or  under 
the  coverglass  on  the  slide.  From  the  mouths  of  such  tubes 
the  molecules  of  the  dissolved  substances  diffuse  in  all  direc- 
tions into  the  surrounding  liquid  and  presently  impinge 
upon  the  motile  organisms  present.  Such  impinging  mole- 
cules set  up  an  irritation  within  the  cell.  When  this  irrita- 
tion is  sufficient,  the  organism  reacts  by  changing  its  posi- 
tion, moving  so  that  its  long  axis  becomes  parallel  to  the 
line  of  advancing  molecules,  and  presently  going  either  to- 
ward or  away  from  the  source  of  the  irritating  substance, 
according  asf  this  substance  attracts  or  repels.  Pfeffer 
found  that  certain  substances  attract,  others  repel,  and  still 
others  induce  no  change  in  the  direction  of  locomotion. 
Pfeffer's  list  has  been  considerably  lengthened  by  one  of  his 
pupils,  Buller,  who  worked  on  the  antherozoids  of  ferns.t 
As  a  result  of  Pfeffer's  work  it  is  now  generally  accepted 
that  the  antherozoids  of  ferns,  liverworts,  and  algae  make 
their  way  to  the  egg-cells  under  the  definite  attraction  of 
soluble  substances  diffusing  from  the  egg-cells  themselves,  or 
from  a  part  or  the  whole  of  the  oogonium  or  archegonium. 
The  specifically  attractive  substances  are  unknown,  though 
it  is  inferred  in  the  case  of  ferns  that  malic  acid  is  the  at- 

*  Pfeffer,  W.  Locomotorische  Richtungsbewegungen  durch  chemische 
Reize.  Unters.  a.  d.  bot.  Inst.  z.  Tubingen.  Bd.  I..  1884.  Uber  chemo- 
taktische  Bewegungen  von  Baktericn,  Flagellaten  und  Yolvocineen.  ibid. 
Bd.  II.  1888.  See  also  Jennings  and  Crosby  on  manner  in  which  bacteria 
react  to  stimuli,  especially  to  chemical  stimuli.  Amer.  Journ.  Physiol.. 
vol.  VI..  1901. 

f  Buller.  A.  H.  R.  Contributions  to  our  knowledge  of  the  physiology  of 
the  spermatozoa  of  ferns.  Annals  of  Botany  vol.  14  1900. 


236  PLANT  PHYSIOLOGY 


tractive  agent,  and  cane-sugar  in  the  liverworts.  This 
inference  is  based  on  experience  only  to  this  extent :  malic 
acid  is  present  in  fern  prothalli  bearing  sexual  organs,  and 
capillary  tubes  containing  a  0.05%  solution  of  malic  acid 
collected  one  hundred  fern  antherozoids  in  an  hour  from  a 
drop  of  water  in  which  there  were  many.  The  evidence  for 
cane-sugar  is  similar.  Presumably  the  free  egg-cells  of 
Fucus,  etc.,  are  fertilized  by  chemotactically  attracted 
antherozoids,  but  proof  of  this  is  wholly  lacking.  Much 
physiological  work  of  this  kind  remains  to  be  done  on  the 
reproduction  of  the  fresh  and  salt  water  algae. 

Pfeffer's  work  on  chemotaxis  was  quantitative  as  well  as 
qualitative.  He  showed  that  Weber's  law,  that  "  the  smallest 
change  in  the  magnitude  of  a  stimulus  which  will  call  forth 
a  response  always  bears  the  same  proportion  to  the  whole 
stimulus,"  applies  to  chemotactic  as  to  other  stimuli.  In 
the  case  of  "Bacterium  termo"—now  known  to  be  a  mixture 
of  several  species  of  short  rod-shaped  bacteria — Pfeffer 
found,  in  the  first  place,  that  there  must  be  more  of  the 
attractive  substance  in  the  capillary  than  outside,  and  then 
that  if  the  hanging-drop  culture  contain  0.01%  meat  extract, 
the  bacteria  will  not  swim  toward  and  into  a  capillary 
containing  less  than  0.03%  and  that  they  are  attracted  more 
and  more  strongly  by  solutions  containing  0.05%,  0.08%, 
and  0.1%.  The  same  organisms  in  a  culture  containing 
0.1%  meat  extract  will  be  attracted  only  by  capillaries  con- 
taining 0.3%,  0.5%,  0.8%,  and  1.0%  solutions  of  meat  extract. 
Similarly,  the  bacteria  are  drawn  from  a  1%  solution  only 
by  a  3%  solution  or  stronger.  We  see  then  that- the  at- 
tractive substance  must  be  increased  by  stages  of  three  in 
order  that  there  may  be  a  change  in  the  direction  of  loco- 
motion. This  does  not  imply  that  there  is  no  stimulation 
with  less,  for  there  may  well  be,  only  that  there  will  not  be 
a  response  in  the  form  of  a  change  in  direction  of  locomo- 
tion. In  order  to  induce  an  organism  in  an  environment  of 
a  certain  degree  of  favorability  to  leave  this  for  another, 
there  must  be  a  decidedly  larger  proportion  of  the  stimulat- 
ing substance. 

Chemotropism  and  chemotaxis  may    be   due   to    funda- 


IRRITABILITY  237 

mentally  the  same  causes,  though  there  is  no  proof  of  this. 
(See  discussion  of  heliotropism  and  heliotaxis  on  page  219. ) 
If  the  chemical  theory  advanced  hi  explanation  of  the  influ- 
ence of  light  upon  the  direction  of  growth  and  of  locomotion 
be  true  (p.  210),  the  influence  of  light  upon  the  living 
protoplasm  is  only  through  those  compounds  in  the  cell 
which  are  acted  upon  by  light.  So  far,  then,  as  the  proto- 
plasm only  is  concerned,  it  may  be  that  heliotropic  and 
heliotactic  phenomena  are  intrinsically  chemotropic  and 
chemotactic.  The  sources  of  the  influence  upon  the  chemical 
compounds  within  the  cell  are  different,  the  influence  which 
directly  affects  the  living  protoplasm  may  be  only  that  of 
chemical  compounds,  not  of  light. 

INFLUENCE  OF  ELECTRICITY 

The  electrical  conditions  in  soil  and  air  are  constantly 
changing,  but  except  at  rare  intervals  no  great  amount  of 
electrical  force  becomes  evident  or  is  concentrated  in  any 
one  place.  Under  natural  conditions  living  organisms  are 
ordinarily  exposed  to  feeble  currents  only,  and  to  such  they 
are  accustomed.  Under  the  artificial  conditions  prevailing 
in  the  planted  streets  and  in  the  parks  of  cities,  in  the  soil 
of  which  powerful  electric  currents  are  induced  or  through 
which  they  are  transmitted  in  consequence  of  the  very 
general  tfse  of  electricity,  the  roots  of  plants  are  peri- 
odically or  constantly  subjected  to  more  violent  electri- 
cal influence.  Not  all  electricity,  however,  comes  from 
without,  for  in  the  plant  itself,  under  normal  conditions, 
electrical  as  well  as  other  currents  are  constantly  developed 
and  maintained.  These  are  very  likely  due  to  the  chemical 
changes  going  on  in  the  plant  and  in  its  different  parts,  and 
just  as  these  processes  affect  the  balance  of  other  forces — of 
heat,  osmosis,  and  chemical  energy — so  they  may  readily 
alter  the  balance  of  electrical  tensions  in  different  parts  of 
the  organism  and  thereby  set  up  electrical  currents.  If, 
by  chemical  and  physical  changes  in  the  organism,  elec- 
trical currents  develop  in  it,  it  is  natural  to  suppose  that 
electrical  influences  brought  to  bear  upon  the  organism 


238  PLANT  PHYSIOLOGY 

from  without  will  effect  chemical  and  physical  changes 
within  it.  These  changes,  whether  in  the  living  protoplasm 
itself  or  in  its  lifeless  contents,  cannot  be  without  influence 
upon  the  protoplasm  itself. 

According  to  Loeb* — "  probably  all  electrical  effects  on 
living  things  are  only  indirect.  What  we  call  electrical 
effects  are  really  due  only  to  the  chemical  and  molecular 
action  of  the  ions  or  the  compounds  formed  from  these" 
set  free  in  the  living  cells,  or  in  the  living  organism,  by 
electrical  force.  Such  an  hypothesis  as  this  is  certainly  com- 
prehensive enough.  Protoplasmic  tensions  and  the  strength 
of  the  protoplasmic  structure  would  surely  vary  with  the 
chemical  and  physical  rearrangements  taking  place  in  the 
dissolved  salts  of  the  cell-sap  throughout  the  living  cell. 

In  spite  of  considerable  experimental  study  of  the  possible 
influence  of  electricity  upon  the  rate  and  amount  of  growth 
no  general  statement  regarding  it  is  now  possible.  Some 
authors  f  claim  that  plants  screened  from  atmospheric 
electricity  are  smaller  than  plants  grown  under  otherwise 
similar  conditions.  Others  J  assert  that  increasing  the  at- 
mospheric electricity  by  discharges  from  wires  strung  above 
the  plants,  increases  both  the  growth  and  the  yield.  The 
eminent  German  agricultural  physicist,  Wollny,§  having 
critically  re-examined  the  subject,  denies  both  claims.  We 
may,  therefore,  safety  leave  this  question  to  the  sanguine 
students  of  applied  botany,  only  hoping  that  the  farmer 
will  not  be  led  to  cumber  the  ground  and  mar  the  land- 
scape with  unsightly  screens  of  telegraph  wire. 

*  Loeb,  J.  Zur  Theorie  des  Galvanotropismus.  Archiv.  f.  d.  ges.  Physio- 
logic, Bd.  67,  1*97. 

t  Grandeau,  L.  De  Finfluence  de  1'electricite  atmospherique  sur  la  nutri- 
tion des  vegetaux.  Ann.  de  Chem.  et  de  Physique.,  T.  XVI.,  1879.  Aloi, 
.A.  DelP  influenza  dell'  elettricita  atmospherica  sulla  vegetazione  delle 
piante.  Bull.  Soc.  Bot.  Italiana,  1895. 

t  Lemstrom,  S.  Om  elektricitens  inflytande  p&  vaxterna.  Helsingfors, 
1890.  Abstract  by  Bailey  in  Trans.  Mass.  Hortic.  Soc.,  1894.  Also  Elec- 
tricultur.  Erhohung  der  Ernteertrage  aller  Culturpflanzen  durch  elek- 
trische  Behandlung.  f  bers.  von  Pringsheim.  Berlin,  1902. 

§  Wollny,  E.  Elektrisch  >  Culturversuche.  Forsch.  a.  d.  Gebiete  d. 
Agrik.-Physik.,  Bd.  XVI.?  1893. 


IRRITABILITY  239 

The  net  result  of  the  many  attempts  to  hasten  the  germi- 
nation or  to  increase  the  germinating-power  of  seeds  by 
subjecting  them  to  electrical  action  seems  to  be  that  there 
are  such  great  differences  between  plants  of  different  species 
that  no  general  rule  can  be  formulated.*  Experiments  on 
the  possible  effects  of  the  X  or  Rontgen  rays  upon  the 
I  growth  of  plants,  from  the  pathogenic  bacteria  up,  have 
led  mainly  to  negative  results. 

It  is  claimed  that  the  direction  of  growth  is  influenced 
by  electrical  currents,  that  plants  are  galvanotropic  or 
electrotropic,  positively  or  negatively,  at  least  as  to  their 
roots.t  If  Loeb's  hypothesis  (p.  238)  be  true  that  electri- 
cal currents  set  up  chemical  changes  in  the  cell,  it  would 
be  surprising  if  these  produced  no  effect  as  shown  by 
the  direction  of  growth.  Still,  the  subject  is  far  from  ex- 
hausted. 

Electrotaxis  in  plants  is  almost  uninvestigated.  Some 
work  on  the  movements  of  bacteria  J  with  relation  to  electri- 
cal currents  has  been  done,  but  with  meagre  results.  Zoo- 
spores  would  suggest  themselves  as  favorable  objects  of 
study. 

INFLUENCE  OF  CONTACT 

By  the  term  contact  is  implied  much  the  same  idea  as  is 
expressed  in  the  physiology  of  higher  animals  and  in  com- 
mon parlance  by  the  word  touch.  When  the  human  finger 
gently  touches  a  motionless  clean  glass  rod,  the  temperature 
and  other  physical  properties  of  which  are  similar  to  those 

*  For  description  of  method  and  the  detail  of  beneficial  effects  of  elec- 
trical stimulation  see  Kinney.  Electro-germination.  Bull.  Hatch  Experi- 
mental Station  Amherst  Mass..  Jan.  1897.  and  Stone.  Influence  of 
Electricity  on  plants.  Bot.  Gazette,  vol.  27.  1899. 

\  Elfving.  F.  Uber  eine  Wirkung  des  galvanischen  Stroms  auf  wachsende 
Keimlinge.  Botan.  Zeitung.  Bd.  40.  1882.  Brunchorts.  J.  Uber  Galvano- 
tropismus.  Ber.  d.  Deutsch.  Bot.  Ges..  Bd.  II..  1884.  and  Bot.  Centralbl., 
Bd.  XXTTT.  1885. 

J  Yerworn.  M.  Die  polare  Erregung  der  Protisten  durch  den  galvani- 
schen Strom.  Arch.  f.  d.  ges.  Physiol..  Bd.  46.  1890.  p.  291.  Lortet,  L. 
I/influence  des  courants  induits  sur  1* orientation  des  Bacteries  vivantes. 
Co.iptes  Rendus  de  1'Acad.  des  Sciences  Nat.,  Paris.  T.  CXXII.,  1896. 


240  PLANT  PHYSIOLOGY 

of  adjacent  objects,  the  mind  is  conscious  of  the  contact. 
Solid  object  and  solid  object  exert  on  one  another  certain 
influences.  The  finger  imparts  a  certain  amount  of 
warmth,  of  moisture  perhaps,  possibly  of  other  forces  and 
substances,  to  the  rod,  and  the  rod  also  acts  upon  the 
finger.  The  contact  of  solid  objects  with  plants  and  plant- 
parts  enables  them  directly  to  influence  each  other.  Only 
when  the  plant  or  plant-part  is  more  responsive  than  the 
lifeless  object,  only  where  there  is  a  visible  reaction  to  the 
influence  thus  exerted,  can  we  tell  either  the  character  or 
the  extent  of  the  influence.  By  studying  the  reactions  to 
the  influence  exerted  by  contact  we  can  divine  something  re- 
garding the  nature  of  the  stimulus. 

When  zoospores  and  other  motile  and  floating  spores  of 
sessile  plants  come  to  rest,  attaching  themselves  to  the  sub- 
stratum, the  attachment  is  effected  through  means  not  yet 
wholly  clear.  The  course  of  events  is  briefly  this.  When  a 
spore,  e.  g.  of  Vaucherm,  (Edogonium,  Fucus,  etc.,  comes  to 
rest  against  the  surface  of  a  solid  object — a  dead  leaf  or 
branch  or  stone — its  foi*m  changes  and  the  naked  mass  of 
protoplasm  invests  itself  with  a  cell-wall.  The  part  of  the 
cell  against  the  solid  object  flattens,  spreads  out  somewhat, 
conforms  accurately  to  the  surface  of  the  object,  and  adheres 
closely  to  it.  In  this  way  the  holdfast  of  the  new  plant  is 
begun.  The  cause  of  this  local  and  peculiar  growth  cannot 
be  due  to  the  stimulation  of  gravitation,  for  such  spores 
attach  themselves  as  frequently  to  oblique  and  vertical  as 
to  horizontal  surfaces.  Nor  can  it  be  warmth  or  moisture, 
or  even  chemical  stimulation.  It  must  be  either  light  or 
contact  or  both.  Light  or  darkness  must  play  a  part  in 
controlling  the  local  growth  of  holdfasts  in  many  plants. 
It  appears,  however,  to  influence  locomotion  and  the  direc- 
tion of  division  of  the  spores*  rather  than  to  stimulate 
growth  in  the  parts  touching  the  solid  object.  We  may 
conclude,  then,  that  contact  with  the  solid  stimulates  the 
protoplasm,  but  it  remains  for  experiment  actually  to  show 
that  the  increased  rate  and  the  changed  direction  of  growth 

*  Winkler,  H.  Einfluss  ausBerer  Factoren  auf  die  Theilung  der  Eier  von 
Cystosira  barbata.  Ber.  d.  D.  Bot.  Ges.,  Bd.  18,  1900. 


IRRITABILITY 


241 


e  19.-Tendrils  of  Ampelopsis  Veitchii. 
*•  before>  ft'  a!ter  atta*hment- 


in  such  cases  among  plants  are  due  to  contact,  though  it 
seems  to  be  the  case  in  animals.* 

Among  higher  plants,  familiar  examples  of  growth  stimu- 
lated by  contact  are  afforded  by  those  species  of  Ampelopsis 
which  form  the  curiously  branched  organs  of  attachment 
shown  in  the  accompanying  figures  (A.  Veitchii).  The 
young  branches,  "ten- 
drils," (a)  are  soft, 
weak,  tapering,  long 
and  slender,  slightly 
enlarged  at  the  tips 
and  green  only  there. 
After  the  tips  come  into 
contact  with  a  suita- 
ble vertical  surface,  they 
broaden  and  flatten  out 
pressing  closely  against 
the  surface,  forming  the  little  discs  which  presently  be- 
come very  strongly  attached  (b).  If  the  "tendrils"  do 
not  touch  a  surface  suitable  for  attachment,  the  tips 
do  not  enlarge,  the  whole  organ  remains  weak,  and 
finally  dies  and  falls  away.  Stability  of  the  support  and 
prolonged  contact  with  it  seem  to  be  indispensable  to  the 
formation  of  the  disc-shaped  bodies.  These  are  the  larger 
the  rougher  the  surface  of  the  support,  other  things  being 
equal.  The  subsequent  thickening  and  strengthening  of  the 
"tendril"  are  due  to  mechanical  pull  (see  pp.  187-8)  rather 
than  to  the  contact  of  the  tips  with  a  rough  object. 

The  direction  of  growth  is  controlled  in  many  instances 
by  contact  with  solid  objects.  Thigmotropism  or  stereo- 
tropism  is  the  name  proposed  for  this  phenomenon.  The 
most  striking  examples  are  furnished  by  tendrils,  f  which 
have  been  studied  by  many  observers. 

The  sensitiveness  of  tendrils  to  contact  varies  greatly 
with  the  species,  the  prevailing  conditions,  and  the  age 

*  Loeb  J.  rntersuchungen  zur  physiologischen  Morphologic  der  Thiere. 
1.  Uber  Heteromorphose.  Wiirzburg,  1891. 

f  See  Darwin's  "  Movements  and  Habits  of  Climbing  Plants"  and   Pfef- 
fer's  P"anzenphysiologie    Bd.  II..  2ter  Theil    2te  Auflage. 
16 


242  PLANT  PHYSIOLOGY 

of  the  tendrils.  They  are  most  sensitive  when  they  are 
half  or  three  quarters  grown  and  until  after  they  have  at- 
tained their  full  length.  A  considerable  degree  of  warmth 
and  humidity  increases  their  sensitiveness,  warmth  being 
the  more  important  factor.  Most  tendrils  are  more  sen- 
sitive on  one  side  than  on  the  others.  The  part  most 
sensitive  to  contact  is  that  between  the  tip  and  middle, 
the  tip  itself  and  the  base  not  being  sensitive  to  contact. 
The  base  is,  however,  sensitive  to  gravity  and  in  this  re- 
spect differs  from  the  rest  of  the  tendril.  The  zones  of 
greatest  sensitiveness  and  of  greatest  growth,  as  in  roots 
(see  p.  201),  are  not  coincident. 

A  tendril  of  P&ssiflora  in  the  right  stage  of  growth  and 
under  favorable  conditions  will  soon  bend  toward  the 
side  touched,  if  gently  stroked  with  a  pencil.  The  extent 
and  duration  of  the  bending  will  be  proportioned  to  the 
degree  of  irritation.  The  rougher  the  surface  of  the  object 
which  the  tendril  touches,  the  more  pronounced  the  bend- 
ing. Because  the  surface  of  tendrils  is  so  smooth,  they  so 
slightly  irritate  each  other  when  they  chance  to  touch  that 
they  rarely  bend  about  each  other.  A  succession  of  light 
touches,  each  too  light  to  induce  a  pronounced  or  per- 
manent bending,  will  stimulate  a  tendril  to  the  same  degree 
as  continuous  contact  with  the  same  object,  but  unless  the 
contacts  are  made  at  very  short  intervals,  so  that  the  ten- 
dril finally  catches  and  entwines  the  stimulating  object, 
thereby  producing  an  enduring  contact  and  a  prolonged 
stimulus,  the  tendril  will  cease  to  bend  more  and  will  finally 
straighten  out.  Presumably  loss  of  sensitiveness  precedes 
loss  of  ability  to  bend,  but  since  the  reaction  is  the  only 
evidence  we  now  have  of  the  sensitiveness,  it  is  impossible 
to  say  positively.  This  experiment  shows,  among  other 
things,  that  there  is  an  accumulation  of  stimuli,  that  the 
response  is  not  immediate  but  induced,  and  that  the  ten- 
dril finally  ceases  to  respond  to  one  degree  of  inconstant 
stimulation,  becoming  either  accustomed  and  unsensitive 
or  else  fatigued  and  unresponsive.  This  condition  has  its 
parallel  in  the  familiar  state  of  the  fatigued  muscle  or 
organism.  A  fresh  horse  lightly  touched  by  the  whip  re- 


IRRITABILITY  243 

spends  to  the  slight  stimulus  by  increasing  his  speed,  but 
the  same  horse  at  the  end  of  a  hard  day's  journey  can- 
not be  made  by  the  same  stimulus,  or  even  by  a  greater, 
to  give  the  same  response.  This  indicates  two  things :  in 
the  first  place,  that  the  touch  of  the  whip  or  the  repeated 
contact  with  a  solid  body  do  not  of  themselves  increase 
the  speed  of  the  horse  or  accomplish  the  bending  of  the 
tendril,  but  that  they  are  merely  stimuli,  that  these  slight 
influences  set  in  operation  other  forces. 

Our  conception  of  the  stimulus  as  merely  the  feeble  force 
which  sets  other  forces  in  operation  is  justified  by  further 
consideration  of  the  behavior  of  horse  and  tendril.  Though 
the  fresh  horse  promptly  responds  to  the  light  touch  of  the 
whip,  there  is  actually  the  lapse  of  some  time  between 
stimulus  and  reaction,  but  this  time  is  brief  because  the 
nerve  and  muscle  systems  of  the  horse  are  exquisitely 
adapted  to  receiving  and  responding  to  such  stimuli.  They 
are  especially  differentiated  to  accomplish  a  few  purposes. 
The  tendril  responds  more  slowly  to  contact.  There  is 
what  is  termecT  the  "latent  period "  between  stimulation 
and  reaction,  ranging  from  five  seconds  or  less  to  an  hour 
or  more  according  to  the  species,  but  during  this  period 
the  forces  set  in  operation  by  the  stimulus  are  working. 

The  means  by  which  a  tendril  curves  around  its  support 
is  not  definitely  known,  and  it  may  well  be  that  the  me- 
chanics of  curvature  are  not  the  same  in  all  cases.  The 
older  plant  physiologists,  like  Sachs*  and  de  Tries,  f  as- 
serted that  the  bending  is  due  to  changes  in  the  rate  of 
growth  on  the  two  sides,  the  side  in  contact  with  the  sup- 
port growing  less  rapidly,  the  opposite  side  more  rapidly, 
than  before  the  contact  was  made.  MacDougalJ  dissents 
from  this  generally  accepted  opinion,  believing  that  there 
are  not  such  changes  in  growth-rate,  but  that  the  side  in 
contact  with  the  solid  object  contracts  decidedly  and  that 

*  Sachs.  J.  von.    Physiology  of  Plants.    Eng.  Edition.  1887. 

f  De  Vries.  H.  Langenwachsthum  der  Ober-  und  Untereeite  krummender 
Ranken.  Arb.  d.  Bot.  Inst.  Wurzburg.  Bd.  I..  1873. 

J  MacDougal.  D.  T.  Mechanism  of  curvature  in  tendrils.  Annals  of  Bot- 
any, X..  1896. 


244  PLANT  PHYSIOLOGY 

the  opposite  side  may  grow  less  rapidly  than  when  the 
tendril  is  free.  Presumably  either  the  contraction  of  the  cells 
on  the  side  touched,  or  the  expansion  ( fixed  by  growth )  of 
the  cells  of  the  opposite  side,  would  develop  the  mechanical 
force  needed  to  accomplish  the  bending,  for  the  resistance 
to  be  overcome  is  slight.  Indeed,  until  they  have  attached 
themselves  to  a  support  and  have  developed  strengthening 
tissues  as  a  result  of  strain  (see  pp.  187-8),  tendrils  are 
mechanically  weak  as  well  as  irritable. 

The  irritability  of  tendrils  varies  greatly,  the  tendrils 
of  grape-vine  requiring  prolonged  contact  with  a  compara- 
tively rough  object,  those  of  passion-vine  ( Passiflora ) 
responding  to  extremely  slight  and  transient  stimuli  in  25 
to  30  seconds.  The  size  of  the  solid  object  must  be  pro- 
portioned to  the  size,  structure,  and  sensitiveness  of  the 
tendril  if  it  is  to  be  successfully  clasped.  The  tendrils  of 
grape,  for  example,  cannot  twine  about  objects  of  small 
size,  while  tendrils  of  Echinocystis  will  coil  about  a  spider's 
thread.*  A  piece  of  thread  weighing  0.00025  milligram, 
placed  as  a  rider  on  a  tendril  of  passion-vine,  causes  no 
stimulus  if  motionless,  but  induces  bending  if  swinging 
on  the  tendril. f  All  solids  except  moist  gelatine  irritate;  no 
harmless  liquid,  free  from  solid  particles,  not  even  mercury, 
stimulates  the  most  sensitive  tendril  to  bend.  From  this 
we  may  infer  that  unless  there  be  a  certain  amount  of 
adhesion  between  the  tendril  and  the  object  by  which  it  is 
touched,  there  will  be  no  irritation.  This  adhesion  changes 
the  pressure  in  the  cells  of  the  part  touched,  increasing 
or  decreasing  the  pressure  according  to  circumstances. 

The  petioles  of  several  well-known  plants  are  more  or 
less  sensitive  to  contact,  e.  g.  Lophospermum  scandens. 
Solanum  j&sminoides,  Trop&olum  majus,  Clematis.  The 
stems  of  dodder  (Cuscuta)  are  also  periodically  sensitive 
to  contact.  %  This  parasite  twines  about  its  host,  forming 

*  MacDougal,  Joe.  cit.,  p.  376. 

f  Pfeffer,  W.  Zur  Kenntniss  der  Contaktreize.  Untersuch.  a.  d.  Bot. 
Inst.  Tubingen,  Bd.  I,  1885. 

I  Peirce,  G.  J.  Contribution  to  the  physiology  of  the  genus  Cuscuta. 
Annals  of  Botany,  VIII.,  1894. 


IRRITABILITY  245 

alternately  the  steep  spirals  typical  of  twining  plants  and 
short  close  spirals  like  tendrils.  After  it  has  sent  haus- 
toria  into  the  tissues  of  its  host,  dodder  elongates  very 
rapidly  for  a  few  hours.  During  this  time  the  stem  is  not 
sensitive  to  contact  and  it  behaves  like  an  ordinary  twining 
plant,  circumnutating  and  obeying  the  force  of  gravity. 
Presently,  however,  the  rate  of  growth  decreases  and  then 
it  begins  to  be  sensitive  to  contact.  When  irritated  by  con- 
tact with  an  object  of  suitable  size,  dodder  will  make  two 
or  three  or  even  more  close  tendril-like  turns .  about  the 
support.  It  will  not  form  these  close  coils  about  moist 
gelatine,  it  cannot  twine  about  objects  too  large  or  too 
small.  The  longer  the  contact  and  the  rougher  the  surface 
of  the  support,  the  more  prompt  and  pronounced  will  be  the 
bending.  Only  prolonged  contact  will  induce  permanent 
bending.  This  is  the  first  effect  induced  by  contact. 

A  second  effect  consists  in  the  formation  of  haustoria, 
lateral  root-like  organs  which  the  parasite  sends  into  the 
tissues  of  its  host  and  through  which  it  draws  needed  food. 
Without  contact  these  organs  never  form,  even  in  rudimen- 
tary conditions.  Without  contact  with  an  object  able  to 
furnish  food  as  well  as  mechanical  support  to  the  parasite, 
the  haustoria  will  not  fully  develop.  Contact  stimulus  in- 
duces the  stem  to  bend  closely  about  the  support  and  to 
form  haustoria,  but  chemical  stimulus  is  needed  besides 
to  secure  the  development  of  the  haustoria.  In  fact,  the 
seedling  dodder  will  not  even  twine  about  innutritions 
supports.  Furthermore,  the  dodder  stem  is  sensitive  to 
gravity  and  will  not  twine  closely  or  otherwise  about  a 
suitable,  even  nutritious,  support  unless  the  position  of 
this  be  vertical  or  only  slightly  inclined. 

The  dodders,  and  a  few  tropical  plants  like  them  ( e.  g. 
Cassytha),  are  exceptional  twining  plants.  The  great 
majority  of  twining  plants  are  not  sensitive  to  contact, 
and  though  they  twine  about  vertical  or  nearly  vertical 
supports  of  suitable  size  and  form,  they  do  so  by  the  com- 
bined action  of  their  spontaneous  nutation  movements 
and  of  gravitation.  As  Darwin  so  clearly  demonstrated,* 
*  Darwin,  C.  The  power  of  movement  in  plants. 


246  PLANT  PHYSIOLOGY 

all  growing  parts,  at  least  of  vascular  plants,  are  in  con- 
stant motion,  owing  to  their  growth  not  being  equal  on  all 
sides  at  any  one  time.  The  rate  of  growth  changes  in  the 
different  parts,  and  because  this  change  in  rate  is  fairly 
regular  and  takes  place  in  adjacent  parts  successively 
around  the  plant  or  part,  the  motion  is  regular,  circular 
or  elliptical  in  a  horizontal  plane,  spiral  in  space.  Darwin 
called  it  circumnutation.  Apparently  the  regularity  of 
the  motion  is  due  to  external  stimuli  rather  than  to  causes 
inherent  in  the  organism.  Experiment*  seems  to  show 
that  the  cooperation  of  the  forces  to  which  plants  are 
constantly  and  successively  subjected — e.  g.  gravitation, 
light,  heat,  etc. — reduces  the  otherwise  wholly  irregular 
movements  to  order  and  system,  and  that  without  these 
stimuli  these  movements  produced  by  unequal  growth  are 
entirely  irregular.  The  circumnutation  of  twining  plants  is 
through  a  somewhat  wider  orbit  than  that  of  other  plants, 
probably  because  of  their  greater  length  in  proportion  to 
their  thickness  and  mechanical  strength.  The  tips  of  the 
stems  and  branches  do  not  stand  out  straight  for  any 
great  distance ;  they  tend  to  droop  somewhat.  The  negative 
geotropism  and  the  ample  nutation  of  the  slender  and 
elongated  stems  of  twining  plants  cause  them  to  grow 
spirally  upward  around  suitable  supports,  t 

Sensitiveness  to  contact  and  later  their  parasitism  were 
probably  acquired  by  dodder  and  its  allies  after  these  plants 
had  developed  the  twining  habit.  The  nearest  relatives  of 
the  dodder  are  still  independent  twiners,  closely  resembling 
the  behavior  of  dodder  in  its  unsensitive  periods.  Further- 
more, under  stress  of  insufficient  food,  the  dodder  is  able  to 
manufacture  some  food  for  itself.  It  will  develop  chloro- 
phyll in  the  usually  rudimentary  and  often  otherwise  col- 
ored chromatophores  which  it  contains  in  large  numbers. 

Thigmotropic  sensitiveness  of  other  organs  and  of  lower 

*  Fritzsche,  Curt.  Uber  die  Beeinflussung  der  Circumnutation  durch 
verschiedene  Factoren.  Inaug.-Diss.  Leipzig,  3  899. 

t  For  a  careful  study  of  the  mechanics  of  twining  see  Kolkwitz,  R. 
Beitrage  zur  Mechanik  des  Windens.  Ber.  d.  D.  Bot.  Ges.,  Bd.  XIII.,  1895. 
The  literature  is  here  cited. 


IRRITABILITY  247 

plants  has  been  demonstrated  by  various  authors,  *  though 
Sachs's  claim  that  roots  are  sensitive  to  contact  seems  to 
have  been  successfully  disproved  by  Newcombe.f  New- 
combe  shows  that  roots  do  not  bend  about  harmless  sub- 
stances ( e.  g.  glass  rods  and  splinters  of  tannin-free  wood ) 
although  they  do  coil  around  pins,  brass  wire,  and  rods 
made  of  wood  containing  tannin  or  other  injurious  mat- 
ters. The  bendings  which  Sachs  described  were  responses 
to  injury  (traumatropic)  rather  than  to  contact.  J  Studies 
of  the  thigmotaxis  of  plants  are  very  few,  if  any  at  all 
exist,  yet  the  behavior  of  such  motile  organisms  as  the 
diatoms,  Oscillatoria,  Beggiatoa,  etc.,  and  the  relations  of 
zoospores  to  the  solid  substances  to  which  they  attach 
themselves,  offer  objects  as  interesting  as  they  are  difficult 
for  investigation. 

Something  must  now  be  said  about  the  so-called  "sensi- 
tive plants."  These  respond  to  a  touch,  a  blow,  or  even  a 
sudden  breath  of  air,  to  a  drop  or  stream  of  water  upon 
the  leaves,  as  well  as  to  changes  in  illumination,  tempera- 
ture, etc.  These  plants  have  compound  leaves,  the  petioles 
of  the  leaves  and  the  stalks  of  the  leaflets  being  supplied 
with  cushion-like  enlargements  called  pulvini,  which  serve  as 
an  articulation  between  petiole  and  blade  or  between  petiole 
and  branch.  A  pulvinus  is  composed  mainly  of  parenchyma 
tissue,  the  cell-walls  of  which  are  elastic  and  readily  per- 
meable to  water.  The  fibre-vascular  bundles  which,  in  the 
petiole  and  in  the  blade  of  the  leaf,  are  separated  from 
one  another  by  plates  of  parenchymatous  tissue,  are  placed 
close  together  in  the  pulvinus  forming  an  axial  strand. 
This  axial  strand  is  the  part  of  the  pulvinus  possessing 
greatest  tensile  strength,  but  the  several  layers  of  paren- 

*  Sachs,  J.  von.  Uber  das  Wachsthum  der  Haupt-und  Nebenwurzeln.  Arb. 
d.  Bot.  Inst.  Wiirzburg.  Bd.  I,  p.  437, 1873,  and  Ges.  Abhandl.  Bd.  II.  p.  826, 
1893.  Errera.  L.  Die  grosse  Wachsthumsperiode  bei  den  Fruchttragern 
von  Phycomyces.  Bot.  Zeitung.  Bd.  42.  1884.  Wortmann,  J.  Zur  Kennt- 
niss  der  Reizbewegungen.  Bot.  Zeitung,  Bd.  45.  1887. 

fNewcombe,  F.  C.  Sachs'  angebliche  thigmotropische  Kurven  an 
Wurzeln  waren  traumatisch.  Beihefte  z.  Bot.  Centralbl,  xii.,  1902. 

£  Spaulding,  V.  M.  The  traumatropic  curvature  oi  roots.  Ann.  of  Bot., 
viii.,  1894. 


248 


PLANT  PHYSIOLOGY 


chyma  cells  by  which  it  is  surrounded  are  what  expand  or 
close,  raise  or  lower,  the  leaflets  and  the  whole  leaf  by 
changes  in  the  amount  of  water  which  they  contain,  in 
other  words,  by  changes  in  their  turgescence.  These  par- 
enchyma cells  act  as  the  cushion  on  which  the  blade 
of  the  leaf  rests.  As  has  been  repeatedly  described  in  the 
text-books,  and  in  numberless  works  on  "the  wonders  of 
Nature/'*  the  position  of  the  leaves  and  leaflets  varies, 
or  may  be  made  quickly  to  change,  according  to  the  con- 
ditions surrounding  the  plant.  The  accompanying  figures 
illustrate  the  periodic  changes  occurring  in  the  position 


Figure  20.  "  Sensitive  Plant ' '  ( Mim osa  pudica )  by  day  ( a ) ,  by  night  ( b ) , 
and  in  light  and  air  of  excessive  brightness  and  dryness  (c) .  From 
MacDougal. 

of  the  leaves  and  leaflets  of  Mimosa  pudica  under  normal 
conditions.  Figure  20  a,  is  the  ordinary  "  day  position,"  when 
the  light  is  fairly  strong,  the  air  warm,  and  water  suffi- 
ciently abundant  to  allow  rapid  transpiration  without  harm. 
Figure  20  b  is  the  ordinary  "night  or  sleep  position/'  when 
the  light  is  weak,  the  air  cool  and  moist  so  that  dew  will 
form.  Figure  20  c  is  the  position  taken  when  the  illumina- 
tion is  so  intense  and  transpiration  so  rapid  that  there 
is  danger  of  excessive  loss  of  water,  t 

*  For  example,  see  "  Living  Plants  and  their  Properties,"  by  Arthur  and 
MacDougal,  Chapter  IV.,  New  York,  1898,  from  which  the  following  de- 
scription is  mainly  drawn. 

t  For  a  discussion  of  these  movements  with  relation  to  transpiration 
eee  pp.  141-2. 


IRRITABILITY 


249 


The  plants  of  Mimosa  pudica  shown  in  the  accompanying 
figures  (21)  were  stimulated  not  by  touch  but  by  a  small 
flame,  though  a  touch  or  blow  of  suffic- 
ient force  would  produce  the  same  ef- 
fect. The  figures  therefore  illustrate 
what  there  is  to  say  of  the  relations  of 
this  plant  to  contact.  Comparing 
these  with  figure  20  a  on  p.  248  we  find 
that  under  favorable  conditions  of 
light,  temperature,  moisture,  etc., 
lightly  touching  one  or  more  of  the  leaf- 
lets of  a  plant  with  the  finger,  a  pencil 
or  some  similar  harmless  instru- 
ment, will  induce  the  leaflets  to  move 
quickly  upward  toward  one  another 
and  slightly  forward,  so  that  they 
come  to  lie  closely  face  to  face  along 
the  leaf-stalk.  An  exceedingly  light 
touch  may  induce  only  one  lea  flee  to 
move,  a  touch  less  light  will  induce 
both  leaflets  of  a  pair  to  move,  one 
still  stronger  will  stimulate  adjacent 
pairs  and  finally  all  the  leaflets  of 
the  compound  leaf  to  close.  At  last 
the  main  petiole  of  the  whole  leaf 
sinks,  bending  at  its  pulvinus.  as  is 
shown  in  figure  21  b.  If  the  touch 
be  sufficiently  strong,  a  blow  rather 
than  a  touch,  the  other  leaves  and 
leaflets  will  behave  in  all  respects 
similarly  ( figure  c )  until  finally  the 
appearance  of  the  plant  will  be  such 
as  indicated  by  figure  d. 

Although  evidently  there  is  still 
much  work  to  be  done  before  the 
subject  will  be  quite  clear,  it  appears 
from  already  published  investiga- 
tions that  the  means  by  which 

Figure  21.  Sensitive  Plant  in 

the  leaves   and   leaflets   are   moved   various  stages  of  stimulation, 
are   found  in  the   parenchyma   cells  From  MacDougal. 


250  PLANT  PHYSIOLOGY 

of  the  pulvini.*  These  cells,  taking  up  water  from  the 
adjacent  vascular  elements  in  the  axial  strand  of  bun- 
dles, expand,  become  turgid,  and  exercise  sufficient  force 
to  raise  the  petiole  or  the  blade  respectively  of  the  leaf 
or  leaflet  above.  The  opposite  effect  follows,  leaflet,  leaf, 
and  petiole  droop  when,  for  any  reason,  the  parenchyma 
cells  of  the  pulvini,  giving  up  the  water  which  they  con- 
tain to  the  vascular  elements,  become  smaller,  flabby,  and 
contracted,  if  not  collapsed. 

A  great  variety  of  stimuli  set  in  motion  the  mechanism 
by  which  the  leaflets  and  leaves  are  closed.  What  is  the  use 
of  these  movements,  and  what  is  the  means  of  transmit- 
ting the  impulse  to  move,  are  by  no  means  clear.  Stahl,f 
from  observations  made  in  the  tropics,  confirms  Darwin's 
claim  that  the  closing  of  the  leaves  is  an  effective  protec- 
tion, in  connection  with  the  thorns  which  the  plant  bears, 
against  hungry  herbivorous  mammals.  It  is  seemingly 
probable  that  contact  and  various  other  stimuli  cause  the 
contraction  of  some  of  the  cells  in  the  leaf  and  that,  in  their 
contraction,  these  cells  expel  water  into  the  vascular  ele- 
ments or  intercellular  spaces.  The  contraction  of  any  cell  or 
group  of  cells  will  necessarily  affect  the  tensions  of  adjacent 
cells.  Thus,  though  we  cannot  conceive  of  the  transmission 
of  the  impulse  itself  from  cell  to  cell,  yet  we  can  readily 
conceive  of  the  extension  to  an  increasing  number  of  cells  of 
the  conditions  first  produced  by  the  irritant  acting  upon  a 
small  number  of  cells.  Mimosa  is  not  able  to  respond  to 
stimulus  (whether  it  is  then  sensitive  or  not  is  another 
question )  when  under  the  influence  of  an  anaesthetic  or  in  a 
state  of  chill,  heat-rigor,  etc.  For  this  reason  the  inference 
is  easy  that  the  physical  changes  induced  by  stimuli  are 

*  Stahl,  E.  Ober  den  Pflanzenschlaf  und  verwandte  Erscheinungen. 
Bot.  Zeitung,  1897.  Cunningham,  D.  D.  The  causes  of  the  fluctuations 
in  the  motor  organs  of  leaves.  Annals  of  the  Botanic  Garden,  Calcutta, 
vol.  VI.,  1895.  MacDougal,  D=  T.  The  mechani  m  of  movement  and 
transmission  of  impulses  in  Mimosa  and  other  "Sensitive"  Plants:  a 
review  with  some  additional  experiments.  Bot.  Gazette,  XXII.,  1896. 
Haberlandt.  G.  Das  reizleitende  Gewebe  der  Sinnpflanze,  Leipzig,  1890. 
Sinnesorgane  im  Pflanzenreich.  Leipzig,  1901. 

f  Stahl,  E.    7.  c. 


IRRITABILITY  251 

transmitted  only  through  living  cells.  This  is  not  neces- 
sarily the  case,  as  MacDougaPs  experiments  tend  to  prove.  * 
Haberlandtf  claims  the  transmission  of  the  stimulus 
through  tubular  series  of  cells  hi  the  phloem  of  the  vascu- 
lar bundles.  Since  Fischer!  has  shown  the  continuity  of  the 
sieve-tubes  throughout  the  plant,  it  is  not  unlikely  that 
Haberlandt's  tubes  are  also  continuous  and  may  therefore 
furnish  at  least  one  course  along  which  the  changed  turgor 
of  any  group  of  cells  could  affect  other  cells. 

The  stamens  of  barberry  and  of  some  of  the  Composite, 
and  the  stigmas  of  Mimulus,  Torenia,  etc.,  contract  on 
being  touched.  Presumably  these,  and  other  similar  move- 
ments, are  due  to  decreased  turgor  in  the  parenchyma  cells 
forming  a  considerable  part  of  the  organ,  as  well  as  in 

(Mimosa. 
CONCLUSION 
So  far  we  have  separately  considered  the  operations  of  the 
more  evident  influences  which  direct  plants  in  growth  and 
in  other  activities.  This  process  of  analysis  leads  us  to 
more  definite  views  regarding  the  effects  of  these  different 
forces  acting  as  stimuli,  but  in  nature  the  plant  is  subjected 
to  all  of  these  forces  more  or  less  constantly  and  more  or 
less  simultaneously.  Thus  light  and  gravitation  may  be 
acting  simultaneously  and,  because  of  the  action  of  both, 
the  response  of  an  organ  to  either  force  will  not  be  the 
same  as  if  that  one  were  acting  alone  (see  p.  214).  Mois- 
ture, warmth,  contact,  and  chemical  substances  may  also 
be  acting  upon  the  plant  at  the  same  time.  The  behavior 
of  a  plant,  then,  expresses  its  adjustment  to  all  the  influ- 
ences operating  upon  it.  Its  size,  form,  color,  vigor,  etc., 
represent  its  response  to  all  the  stimuli  it  has  received. 

Furthermore,  as  conclusively  proved  by  recent  experi- 
ment^ whatever  stimulates  or  otherwise  affects  one  part  or 

*  MacDougal,  D.  T.  7.  c.  t  Haberlandt.  G.    7.  c. 

J  Fischer,  A.  ^eue  Beitrage  zur  Kenntniss  der  Siebrohren.  Ber.  d.  math.- 
phys.  Classe  d.  K.  Sachs.  Ges.  d.  Wiss.,  1886. 

§  Hering,  F.  f  ber  Wachsthumscorrelationen  in  Folge  mechanischer  Hem- 
mung  des  Wachsens.  Jahrb.  f.  wiss.  Bot..  Bd.  29,  1896.  Kny,  L.  Corre- 
lation in  growth  of  roots  and  shoots.  II.  Ann.  of  Bot..  XV.,  1901.  Other 
papers  cited  by  these  authors,  and  by  Pfeffer  Pflanzenphysiologie  Bd.  II., 
Theil  1  1901. 


252  PLANT  PHYSIOLOGY 

organ  affects  all  the  other  parts  of  the  plant.  Injury  or 
loss  of  a  part  is  always  followed  in  healthy  plants  by  the 
replacement  of  the  part,  either  by  new  tissues,  or  by  an- 
other part  assuming  the  work  of  that  injured  or  lost.  Thus, 
if  the  tip  of  a  pine  or  other  perennial  with  excurrent  stem  j 
is  injured,  the  lateral  branches  change  their  direction  of 
growth,  and  finally  the  strongest  and  most  rapidly  growing 
of  these  assumes  the  direction  and  functions  of  the  main 
stem.  "Cutting  back"  stem  or  root  is  followed  by  copious 
branching.  When,  as  in  floating  specimens  of  Marsilia,  etc., 
enough  water  is  absorbed  by  other  parts,  the  roots  soon 
cease  to  grow  and  may  even  finally  disappear. 

We  must  conclude,  then,  that  the  plant  is  sensitive  as  a 
whole  because  of  the  sensitiveness  of  its  parts,  that  the 
condition  of  one  part  affects  all  the  other  parts,  that  cor- 
relation is  a  necessity  if  the  organism  is  to  act  as  an  in- 
dividual or  even  as  an  association  of  cooperating  members. 

Besides  the  forces  which,  through  analysis,  we  have  been 
able  to  recognize  distinctly,  there  are  complex  influences 
consisting  of  forces  and  influences  which  have  so  far  eluded 
analysis.  The  analytical  method  has  not  yet  exhausted  the 
subject ;  more  detailed  physical  and  chemical  knowledge  will 
come  presently.  Meantime  it  is  more  or  less  the  fashion, 
under  the  name  of  ecology,  to  view  things  in  the  large  way, 
and  by  feeling  rather  than  by  trt  application  of  exact 
physiological  methods,  to  reach  conclusions  regarding  the 
effects  of  environment  and  of  association.  The  trees  of  a 
given  species,  presenting  one  appearance  when  they  grow  as 
members  of  a  thick  forest,  are  very  different  when  growing 
separately  in  the  open.  These  differences  in  appearance  are 
due  in  great  part  doubtless  to  differences  in  the  amounts  of 
light  and  food,  of  mechanical  strain,  and  of  room,  but  these 
are  not  all,  nor  do  we  know  the  relative  importance  of  the 
separate  influences.  We  do  not  know  why  small  plants  of 
characteristic  species  are  the  regular  associates  of  certain 
kinds  of  trees.*  Such  phenomena  show  that  plants  are 

*  For  example,  see  Hock,  F.  Begleitpflanzen  der  Kiefer  in  Norddeutsch- 
land.  Ber.  d.  Deutech.  Bot.  Ges.,  Bd.  XI.,  1893;  Coulter,  Plant  Relations. 
New  York,  1899,  etc. 


IRRITABILITY  253 


sensitive  to  all  the  forces  and  influences  which  we  combine 
without  analysis  under  the  name  environment,  and  that  the 
cooperation  of  these  influences  induces,  as  i-eactions  in  the 
living  plants,  the  qualities  which  we  see  and  call  character- 
istic of  the  species,  order,  or  class — reactions  which  are 
realty  characteristic  only  of  the  living  protoplasm.  Proto- 
plasm is  not  all  equally  sensitive  to  any  one  influence  or  to 
the  whole  complex  of  influences  which  constitute  its  living 
and  lifeless  environment.  These  different  degrees  of  sensi- 
tiveness, coupled  with  different  powers  of  response,  are  what 
bring  about,  in  the  same  environment,  the  forms,  sizes? 
colors,  etc.,  which  characterize  individuals  and  species. 

The  various  forces  operating  upon  the  living  organism  set 
up  and  maintain  in  it  physical  and  chemical  conditions 
which  can  be  changed  only  by  the  introduction  of  a  new 
force  or  by  a  change  in  the  relative  proportions  of  the  old 
forces.  The  living  differs  from  the  dead  organism  and  from 
all  lifeless  compounds  and  structures  in  the  supreme  deli- 
cacy of  its  structure,  due  to  the  arrangement,  complexity, 
and  instability  of  the  compounds  composing  and  contained 
within  it  (see  pp.  183-6).  The  sensitiveness  of  living  or- 
ganisms is  different  in  degree  and  not  in  kind  from  that 
of  lifeless  things;  the  sensitiveness  of  both  is  a  matter  of 
physics  and  chemistry. 


CHAPTER    VII 

REPRODUCTION 

THE  subject  of  reproduction  has  been  more  fully  and  more 
exactly  studied  by  morphologists  than  by  physiologists. 
It  has  been  meditated  upon  more  than  it  has  been  investi- 
gated through  experiment.  Yet  there  are  certain  results  of 
comparatively  recent  work,  and  there  are  certain  hypothe- 
ses, which  must  be  considered  in  any  discussion  of  the 
physiology  of  plants. 

In  the  vegetable  as  in  the  animal  kingdom  the  span  of 
life  of  the  individual  organism  is  limited.  In  many  cases  it 
is  limited  by  perfectly  obvious  influences;  in  most  it  is 
limited  by  means  little  understood  even  if  apprehended  at 
all.  In  many  plants  and  animals  there  are  no  evident  rea- 
sons inherent  in  the  organisms  themselves  why  they  should 
not  continue  to  live  indefinitely.  Influences  wholly  external 
and  only  slightly  controllable  by  the  organism  determine 
and  terminate  its  career.  The  effects  of  these  influences  are 
to  be  distinguished  from  the  irritable  responses  which  we 
have  studied  in  the  preceding  chapter.  Irritable  response 
depends  upon  a  degree  of  sensitiveness  possessed  only  by  the 
living  organism,  although  this  sensitiveness  is  dependent 
upon  the  sum  of  the  physical  and  chemical  conditions  pre- 
vailing in  the  organism  (p.  186).  But  the  influences  which 
terminate  its  career  exceed  the  powers  of  resistance,  reac- 
tion, or  response,  of  the  organism.  They  act  upon  it  as 
upon  any  lifeless  thing  of  similar  composition  and  structure, 
and  they  produce  on  the  lifeless  similar  effects  to  those  pro- 
duced on  the  living  body.  For  example,  the  heavy  wind 
which  uproots  a  tree  would  bring  it  to  the  ground  were  it 
alive  or  dead.  Uprooted  it  would  dry  faster  than  if  its 
roots  were  still  in  the  soil.  It  is  after  all  the  drying,  follow- 
ing the  uprooting,  and  not  the  uprooting  itself,  which  is  the 


REPRODUCTION  255 

fatal  thing.  Again — the  frost  which  kills  a  living  herb 
would  produce  in  another  similar  but  lifeless  herb  the  same 
injurious  mechanical  changes,  which  neither  living  nor  lifeless 
herb  could  obliterate  or  reverse  and  recover  from.  Ex- 
cessive shade  or  light  injure  and  may  be  fatal  to  living 
plants,  the  former  entailing  insufficient  food-manufacture, 
the  latter  causing  undue  activity  in  this  or  in  other  ways. 
Lifeless  structures  as  sensitive  to  light  will  also  be  affected 
by  the  same  means;  the  photographic  plate  is  injured  or 
ruined  by  insufficient  or  by  excessive  light. 

From  these  examples  the  inference  is  obvious  that,  if  the 
conditions  which  make  life  possible  ( p.  6 )  were  maintained, 
many  organisms  which  now  die  at  the  end  of  a  season 
or  a  cycle,  would  continue  to  live.  The  inference  could 
not,  however,  be  extended  to  all  plants,  because  what  de- 
termines the  span  of  life  of  the  organism  is  often  within  it, 
not  outside.  This  every  one  knows.  Wheat  harvest  comes 
long  before  frost  or  heat  or  drought  can  terminate  the  lives 
of  the  wheat  plants.  In  California  the  wheat  plants  are 
dead  before  harvest  begins.  They  have  ceased  to  live  when 
they  have  matured  their  fruits.  They  have  transferred  to 
the  embryos  in  the  fruits  the  life  which  they  themselves 
possessed.  The  one  living  wheat  stalk  has  formed  several 
or  many  kernels,  each  containing  a  living  plantlet.  Among 
these  new  individuals  the  life  of  the  parent  has  been  com- 
pletely distributed.  The  parent  stalk  ceases  to  live,  its  life 
is  ended,  the  parent  lives  only  in  its  offspring.  No  external 
influences  have  contributed  to  the  death  of  the  stalk  except 
as  they  have  contributed  to  its  successful  life  and  to  its 
production  of  successors.  So  it  is  with  other  plants  which 
fruit  once  and  then  die,  whether  fruiting  take  place  in  a 
month,  a  season,  or  a  "century."  By  preventing  fruiting, 
man  can  artificially  prolong  the  life  of  many  such  plants. 
The  flowering  plants  of  our  gardens  are  induced  to  continue 
blooming  and  to  live  longer  by  being  kept  from  setting 
seed.  By  constantly  picking  the  flowers  of  sweet-pea,  pansy, 
etc.,  larger,  handsomer  and  more  numerous  flowers  may  be 
had  for  a  longer  time  than  if  the  life  of  the  individual  plant 
were  allowed  to  pass  over  into  the  new  individuals  repre- 


256  PLANT  PHYSIOLOGY 

sented  by  the  embryos  in  the  seeds.  The  life  cannot  pass 
over,  however,  without  a  physical  basis;  the  substance  of 
the  parent  is  given  to  and  forms  the  offspring.  The  giving 
of  substance  and  of  life  are  simultaneous  if  not  identical. 
When  this  gift  exceeds  a  certain  amount,  the  parent  ceases 
to  be,  it  abandons  its  own  body,  it  lives  only  in  its  off- 
spring. Internal  causes  are  what  terminate  the  lives  of 
plants  like  these,  but  as  we  shall  presently  see  (pp.  263- 
76),  these  are  profoundly  affected  by  external  influences. 

There  are  other  internal  causes  which  may  terminate  the 
life  of  an  individual.  If  its  parent  endow  it  so  badly  with 
substance,  form,  or  food  that  it  cannot  maintain  the  bal- 
ance of  constructive  and  destructive  processes  in  which  liv- 
ing consists,  it  will  die.  Its  span  of  life  is  determined  by  its 
own  substance,  structure,  and  energy,  all  of  which,  or  the 
means  of  gaining  which,  were  given  it  by  its  parents.  Such 
an  individual,  unable  to  live  long  enough  to  produce  off- 
spring, ceases  to  live  because  its  internal  balance  breaks. 
Its  life  ceases  because  destructive  processes  exceed  the  con- 
structive. Its  lifeless  substance  thereupon  becomes  the 
source  of  matter  and  of  energy  for  living  individuals  of 
other  kinds. 

Since  living  may  be  said  to  consist  in  maintaining  the 
balance  between  construction  and  destruction,  life  may  be 
said  to  represent  the  balance  of  energy-storing  and  energy- 
liberating  processes.  There  is  as  much  substance  and  as 
much  energy  when  there  is  no  balance.  When  the  balance  is 
attained,  there  is  life;  when  the  balance  is  not  attained, 
there  is  no  life.  When  an  individual  dies  without  offspring, 
there  is  less  life  in  the  world  but  no  less  substance  or  energy. 
When  an  individual,  having  given  sufficient  substance  in 
suitable  form  to  its  offspring  to  start  them  in  their  careers, 
continues  to  live  for  a  time,  there  is  less  life  when  it  dies 
but  no  less  matter  or  energy.  There  is  simply  a  different 
adjustment  of  forces,  a  different  arrangement  of  matter. 
But  this  different  adjustment  is  merely  local,  more  or  less 
individual.  The  population  of  a  given  locality  may  greatly 
increase,  but  with  the  increase  in  number  of  human  beings 
there  is  a  decrease  in  the  number  of  living  organisms  of 


REPRODUCTION  257 

Certain  other  sorts.  The  multiplication  of  the  individuals  of 
one  species  may  often  result  in  an  increase  in  the  number 
of  living  organisms  in  the  locality,  but  there  is  no  evidence 
that,  as  a  whole,  the  living  population  of  the  earth  is  in- 
creased. Keproduction  is  not  a  process  by  which  more  is 
made  out  of  a  given  amount,  for,  if  this  were  so,  the  be- 
ginning would  be  the  making  of  something  from  nothing. 
Force  and  matter  remaining  constant,  reproduction  can 
only  maintain,  it  cannot,  beyond  a  certain  point,  increase 
the  total  number  of  living  organisms. 

We  are  thus  led  to  see  that  reproduction  is  a  process  of 
which  the  chief  end  is  maintenance  and  increase  of  the  spe- 
cies rather  than  the  increase  in  life.  The  command  "Be 
fruitful  and  multiply"  is  coupled  with  the  law  which  irre- 
sistibly forbids  increase  beyond  a  certain  point.  As  char- 
acteristic of  the  living  substance  as  breathing  or  feeding  is 
the  tendency  to  maintain  and  to  perpetuate  itself.  Only  by 
securing  more  supplies  of  matter  for  replacing  worn-out 
parts,  and  of  energy  for  carrying  on  its  functions,  does  the 
living  organism  maintain  itself.  When  it  secures  more  mat- 
ter than  is  needed  to  repair,  and  more  energy  than  is 
needed  to  operate,  already  existing  parts,  it  grows.  WTien 
for  any  reason  growth  beyond  a  certain  characteristic  size 
is  impossible  or  difficult,  while  the  supply  of  matter  and 
energy  continues  the  same,  there  must  be  some  other  mode 
of  increase  of  the  individual,  whether  cell  or  organism. 
The  one  individual,  cell  or  organism,  after  forming  new  sub- 
stance (protoplasm)  and  furnishing  it  with  energy  (heat, 
etc.),  may  finally  separate  this  as  a  new  individual.  The 
mother  cell  divides  it  off  as  a  new  cell,  the  parent  organism 
separates  it  as  a  reproductive  body,  a  sperm  or  egg,  a 
spore  or  seed,  an  embryo  or  a  larva.  Reproduction  has 
been  defined  as  "Growth  beyond  the  limits  of  the  indi- 
vidual." Like  growth,  it  depends  upon  the  successful  per- 
formance of  the  ordinary  functions,  and  it  simply  insures 
their  continuance. 

Again,  reproduction  is  said  to  be  "the  effort  to  bridge  the 
gap  of  death."  This  definition  is  suggestive  but  misleading. 
By  reproduction  living  is  continued ;  the  life,  the  substance, 


258  PLANT  PHYSIOLOGY 

and  the  activities,  of  the  offspring  were  those  of  the  parents. 
There  is  no  gap  or  break,  there  is  perfect  continuity.  If  a 
gap  were  to  occur,  nothing  could  bridge  it,  there  would  be 
nothing  with  which  to  bridge  it,  the  species  or  the  race 
would  be  extinct.  A  new  creation  would  be  necessary,  and 
experience  does  not  encourage  belief  in  new  creations.  Re- 
production, then,  prevents  the  formation  of  a  gap.  When 
death  comes  to  an  organism  which  has  already  formed  a 
new  individual  consisting  of  and  continuing  the  substance, 
structure,  and  activities  of  the  old,  death  effects  no  break, 
there  is  no  gap.  When  death  comes  to  an  organism  which 
has  not  yet  grown  beyond  itself,  a  gap  is  formed,  but 
formed  too  soon  ever  to  be  bridged  or  closed. 

The  chief  end  of  reproduction  is,  then,  the  maintenance  of 
life,  the  continuity  of  the  species.  Another  end  is  only  less 
important,  that  of  increasing  the  number  of  individuals. 
The  parent  stalk  of  wheat  which  has  given  all  its  living 
substance  to  the  two  dozen  or  so  new  individuals  enclosed 
within  the  kernels  it  has  produced,  is  contributing  to  in- 
crease the  number  of  wheat  individuals  living  next  year.  As 
we  know,  there  will  be  no  real  increase,  however,  unless 
these  kernels  fall  into  spots  where  no  plant  or  only  a  weaker 
one  lived  before.  Under  natural  conditions,  which  have  so 
long  remained  the  same  that  the  possibilities  of  the  situa- 
tion are  as  fully  exploited  by  the  organisms  living  there 
now  as  can  be  the  case  at  the  present  stage  in  evolution, 
there  will  be  no  vacant  spots  in  which  an  increased  number 
of  individuals  can  live.  Under  these  conditions  reproduction 
cannot  effect  an  increase,  it  can  only  continue  the  life,  of  the 
species.  However,  when  any  new  factor  is  introduced,  when 
seismic  disturbance,  or  dimatic  change,  or  the  entrance  of 
some  new  arid  powerful  organism,  modifies  the  conditions, 
each  new  individual  in  a  brood  or  crop  has  a  different 
chance  from  before,  a  better  chance  or  a  worse  according  to 
the  relative  characters  of  the  new  individual  and  of  the 
changed  environment.  Those  organisms  which  have  young 
ready  and  able  to  take  advantage  of  the  changed  environ- 
ment will  thereby  have  a  better  chance  to  increase  their 
kind  as  well  as  to  maintain  it.  As  the  maintenance  of  the 


REPRODUCTION  259 

species  is  always  more  important  than  its  increase,  and  as 
the  increase  of  the  species  is  only  sometimes  possible,  such  a 
system  of  reproduction  as  will  best  serve  the  chief  end  has 
been  developed  by  every  successful  organism.  Many  or- 
ganisms possess  one  means  of  reproduction  which  combines 
these  two  ends,  others  have  different  means  leading  to  the 
two  ends  separately.* 

Two  modes  of  reproduction,  almost  universal  in  their 
occurrence  among  plants,  can  be  distinguished,  the  sexual 
and  the  non-sexual.  In  the  former,  two  cells  from  two 
different  sources  unite  and  thereby  form  a  new  individual. 
In  the  latter,  the  new  individual  is  developed  from  one  cell 
or  from  a  group  of  cells.  The  difference  between  these  two 
would  seem  very  clear  were  it  not  for  the  fact  that  it  some- 
times happens  spontaneously  in  nature,  and  may  be  made 
to  happen  in  a  considerable  number  of  cases  in  the  labora- 
tory, that  one  of  the  sexual  elements  (cells)  develops  into 
a  new  individual  without  first  fusing  with  the  other  sexual 
element  (cell).  The  development  of  one  sexual  cell  into  a 
new  individual  without  first  fusing  with  the  other  sexual  cell 
is  called  parthenogenesis.  Parthenogenesis  is,  in  effect,  the 
same  as  non-sexual  reproduction.  Morphologically,  sexual 
reproduction  differs  from  non-sexual  reproduction  in  the 
fusion  of  two  cells  into  one.  Physiologically,  sexual  repro- 
duction differs  from  non-sexual  reproduction  in  the  causes 
which  lead  to  it  and  in  the  results  produced  by  it.  In 
sexual  reproduction,  one  cell  is  "  fertilized"  by  the  fusion 
with  it  of  the  other  sexual  element.  In  non-sexual  reproduc- 
tion the  cells  are  "fertile"  without  this  fusion.  "Fertiliza- 
tion" has  been  regarded  as  giving  the  stimulus  needed  by 
the  sexual  cell  for  development  as  a  new  individual.  Non- 
sexual  reproductive  cells  do  not  require  this  stimulus  to 
develop  as  new  individuals.  In  natural  parthenogenesis,  the 
one  sexual  cell,  the  egg-cell,  also  develops  as  a  new  indi- 
vidual without  the  stimulus  of  fertilization.  In  partheno- 
genesis artificially  produced  in  the  laboratory,  chemical  or 
other  stimuli  applied  to  the  eggs  cause  them  to  develop  as 

*  What  these  means  are.  may  be  learned  from  Campbell's  University 
Text  Book  of  Botany.  New  York,  1902. 


260  PLANT  PHYSIOLOGY 

new  individuals.*  So  far,  individuals  thus  produced  have 
not  developed  to  maturity,  though  this  may  be  expected  to 
be  attained.  There  is,  however,  an  essential  difference  be- 
tween individuals  produced  on  the  one  hand  by  non-sexual 
means,  by  natural  parthenogenesis,  and  from  sexual  cells 
artificially  stimulated,  and  on  the  other  hand  by  sexual 
means— the  fusion  of  two  cells.  In  the  latter  case,  the  one 
cell  absorbs  not  only  simple  inorganic  compounds,  but  also 
all  the  complex  organic  compounds  contained  in  the  other 
sexual  element ;  the  structure  as  well  as  the  substance  of  the 
two  cells  unites ;  two  sets  of  organs  are  combined  into  one ; 
nucleus  fuses  with  nucleus,  cytoplasm  with  cytoplasm; 
the  individual  characters  of  the  two  cells  are  combined  in 
the  new  individual.  We  see,  then,  that  fertilization  is  more 
than  stimulation ;  it  is  union. f  In  this  union  of  structure, 
organs,  and  characters,  although  these  are  composed  of 
chemical  compounds,  the  definite  arrangement  of  the  parts 
in  the  sexual  elements  is  as  essential  as  the  composition  of 
the  parts.  The  result  in  the  new  individual  will  vary  with 
the  arrangement  of  the  parts  in  each  sexual  element.  In 
other  words,  there  is  not  only  a  chemical  basis  to  fertiliza- 
tion; there  is  also  a  mechanical.  Fertilization  cannot  be 
complete  and  perfect  without  both  chemical  and  mechanical 
effects.  What  the  significance,  and  what  the  relative  im- 
portance, of  the  chemical  and  mechanical  factors  in  fertili- 
zation may  be  is  now  the  subject  of  hypothesis  and  experi- 
ment. The  matter  is  far  from  settlement. 

That  there  are  advantages  in  a  mode  of  reproduction 
which  unites  chemical  and  mechanical  effects,  and  in  develop- 
ment which  follows  only  after  these  are  produced,  would 
seem  to  be  indicated  by  the  development  of  sexual  repro- 
duction by  so  many  different  types  of  organisms.  If  non- 

*  Loeb.  J.  Artificial  production  of  normal  larva?  from  the  unfertilized 
eggs  of  the  Sea  Urchin  (Arbacia).  Amer.  Journ.  Physiology,  vol.  III., 
1900.  Experiments  on  artificial  parthenogenesis  in  Annelids  (Chaetop- 
terus).  Ibid.  vol.  IV.,  1901.  Nature  of  the  process  of  fertilization.  Ibid. 
vol.  IV.,  1901.  Matthews,  A.  P.  Artificial  parthenogenesis  produced  by 
mechanical  agitation.  Amer.  Journ.  Physiol.,  vol.  VI.,  1901. 

t  Strasburger,  E.  f  ber  Befruchtung.  Bot.  Zeitung,  1901.  Boveri,  Th. 
Das  Problem  der  Befruchtung.  Jena,  1902. 


REPRODUCTION  261 

sexual  reproduction  were  the  more  successful  mode  of  repro- 
duction, would  its  relative  importance  be  so  greatly  reduced 
among  higher  organisms,  both  plants  and  animals?  Yet 
what  the  advantages  of  sexual  over  non-sexual  reproduction 
are,  is  by  no  means  certain. 

Between  animals  and  plants  there  are  striking  resem- 
blances in  the  processes  which  constitute  and  succeed  sexual 
reproduction.  Fertilization  and  the  formation  of  new  indi- 
viduals are  the  same  in  the  two  kingdoms,  and  the  care  of 
the  young  between  the  times  of  their  conception  in  the  body 
of  the  mother  and  of  their  separation  from  it  also  cor- 
respond. Only  among  the  highest  animals,  the  Mammalia, 
is  the  care  of  the  offspring  carried  beyond  what  is  found 
among  plants.  The  plant-mother  cannot  nourish  her  off- 
spring after  it  has  been  separated  from  her  body,  though  she 
has  supplied  it  with  a  store  of  food.  The  mammalian  mother 
can  and  does.  This  difference  of  degree  in  development 
does  not,  however,  make  our  comparison  less  suggestive. 

One  point  more  needs  emphasis  in  this  connection.  Since 
the  continuity  of  the  species  is  the  chief  end  of  reproduction, 
precautions  to  ensure  its  attainment  must  be  taken.  The 
lower  plants  like  the  lower  animals,  simple  in  structure  and 
small  in  size,  as  a  rule  rely  on  the  survival  of  some  of  their 
numerous  and  but  slightly  developed  offspring  rather  than 
upon  the  better  equipment  of  a  smaller  number  of  more 
completely  developed  young.  Among  higher  organisms,  the 
opposite  is  the  general  rule.  The  relatively  small  number  of 
the  seeds  of  the  Leguminosse,  each  seed  containing  a  large, 
well  fed,  and  highly  developed  embryo,  may  be  regarded  as 
typical  of  the  higher  plants,  while  the  small,  not  fully  de- 
veloped embryos  of  the  Orchidaceae,  Ericaceae,  etc.,  in  the 
seeds  of  which  only  small  quantities  of  food  are  stored, 
represent  a  distinctly  lower  type. 

There  are  two  kinds  of  non-sexual  reproduction,  the  vege- 
tative and  the  spore.  In  the  former  a  mass  of  tissue,  capable 
of  carrying  on  many  functions  and  of  developing  into  a  new 
plant,  is  separated  from  the  single  parent.  In  the  latter, 
a  single  cell  or,  at  most,  two  cells  together  are  cut  off. 
These,  though  formed  sometimes  by  the  cooperation  of 


262  PLANT  PHYSIOLOGY 

many  cells,  are  never  formed  by  the  fusion  of  cells.  Each 
of  the  cells  cut  off  is  able,  by  successive  divisions,  to  form 
a  new  plant  like  its  parent.  The  one  parent  or  parent-cell 
or  mass  of  cells  gives  its  own  matter  and  energy,  its  own 
protoplasm,  to  the  new  individual.  Because  the  substance 
of  only  one  parent  goes  into  the  offspring  there  is  a  radical 
difference  between  this  non-sexual  mode  of  reproduction  and 
the  sexual  mode.  With  the  transfer  from  parent  to  off- 
spring of  its  own  substance  and  means  of  obtaining  energy 
there  are  transferred  also  the  same  degree  of  sensitiveness, 
the  same  powers  of  reaction  to  stimuli,  and  the  results  of 
the  reaction  to  stimuli  already  accomplished  by  the  living 
protoplasm  of  the  parent.  The  offspring  are  not  only  like 
the  parent,  they  are  really  branches  or  continuations  of 
the  parent.  There  is  introduced  into  the  new  individual 
nothing  new,  but  as  soon  as  it  is  cut  off  from  the  parent, 
it  is  subjected  to  influences  some  of  which  may  be  new 
and  different  and  which  will  stimulate  the  individual  to 
corresponding  reactions.  The  gemmules  of  certain  Mar- 
chantiaceae,  the  lateral  branches  which  gradually  become 
the  separate  plants  of  Azolla,  the  runners  of  the  straw- 
berry, etc.,  are  means  of  non-sexual  (and  vegetative)  repro- 
duction. The  matter  and  energy,  the  substance  and  struc- 
ture, of  the  one  parent  are  carried  directly  over  into  these 
offspring.  The  condition  of  the  parent  at  the  time  of  re- 
production, representing  the  resultant  of  all  its  reactions 
to  all  the  stimuli  to  which  it  has  been  subjected,  is  also 
the  condition  of  offspring  made  in  this  way.  This  fact  is 
still  clearer  in  such  a  plant  as  Spirogyra,  which,  breaking 
up  into  the  cells  of  which  the  filament  is  composed,  *  forms 
new  individuals  which  share  its  qualities  by  sharing  its  sub- 
stance and  structure.  In  many  other  low  plants  the  proc- 
ess of  vegetative  reproduction  is  as  simple  and  clear  as 
possible.  Similar  but  multicellular  bodies  vegetatively  re- 
produce higher  plants,  continuing  and  at  times  multiply- 
ing the  species  with  nearly  or  quite  the  same  characters 
in  the  new  as  in  the  old  individuals.  The  runners  of  straw- 

*  Benecke,  W.    Mechanismus  und  Biologie  des  Zerfalles  der  Conjugaten- 
faden  in  die  einzelnen  Zellen.    Jahrb.  f.  w.  Bot.,  32,  1898. 


REPRODUCTION  263 

berry,  the  suckers  of  lilac  and  Sequoia,*  the  stolons,  off- 
sets, runners,  etc.,  of  other  plants,  illustrate  this  matter. 

In  the  spore  mode  of  non-sexual  reproduction  single  cells 
or  pairs  of  cells,  each  capable  of  forming  a  new  plant,  are  cut 
off  by  the  parent.  These  reproductive  bodies,  like  the  vege- 
tatively  reproductive  bodies  above  mentioned,  contain  and 
continue  the  substance  and  the  condition  of  the  one  parent. 
So  soon  as  they  are  separated  from  the  parent,  they  may 
be  so  influenced  by  the  factors  of  their  environment  as  to 
depart  from  the  character  of  their  parent,  to  vary. 

The  new  individuals  produced  by  non-sexual  means  are 
usually  more  numerous  than  those  produced  sexually.  Non- 
sexual  reproduction  is  especially  the  means  by  which  the 
species  is  multiplied.  Many  species,  however,  have  only  one 
means  of  reproduction,  hence  the  two  ends,  of  maintaining 
and  of  multiplying  the  species,  are  met  by  the  same  means. 
Many  organisms,  however,  endowed  with  both  means  of 
reproduction,  employ  one  at  one  time,  another  at  another 
time.  What  determines  the  organism  in  its  behavior?  And 
what  are  the  advantages  of  each  mode?  The  first  ques- 
tion is  answerable  at  the  present  time  through  the  already 
available  means  of  experimenting;  the  second  is  not  defi- 
nitely answerable  now,  though  the  interpretation  of  what 
is  observed  in  nature,  and  speculation  partially  supported 
by  experiment,  suggest  probable  answers.  From  (Edogomum 
and  Coleoch&te  among  the  green  algae  to  the  highest  of  the 
flowering  plants,  the  phenomenon  of  alternating  generations 
is  one  of  the  most  conspicuous  in  the  life  of  plants.  How 
this  has  originated  and  the  influences  which  now  control  it 
constitute  one  of  the  most  intricate  problems  in  morphol- 
ogy and  physiology,  the  solution  of  which  remains,  how- 
ever, hidden  in  the  future. 

The  preceding  chapter  has  shown  us  that  the  size,  form, 
color,  rate  of  growth,  direction  of  growth  and  of  move- 
ment, and  many  other  characters,  represent  the  reaction  of 
the  organism  to  the  external  influences,  its  response  to  the 
stimuli,  which  are  collectively  termed  its  environment.  We 

*  Peirce,  G.  J.  Studies  on  the  coast  redwood.  Proc.  Cal.  Acad.  Sciences. 
3d  series,  Botany,  vol.  II..  1901. 


264  PLANT  PHYSIOLOGY 

are,  therefore,  inclined  to  believe  that  its  reproduction  and 
its  reproductive  processes  may  also  be  similarly  affected. 
Yet  this  belief  is  so  modern  as  to  be  almost  heretical.  The 
morphology  of  reproduction  has  been  studied  by  many; 
very  few  have  engaged  in  experimental  studies  of  the  phys- 
iology of  reproduction  among  plants.  Of  these  few  Sachs, 
Vochting,  and  Klebs  merit  first  mention.  These  men  have 
shown  that  upon  external  influences  quite  as  much  as  upon 
so-called  inherited  impulses  depend  the  various  kinds  and 
stages  of  reproduction. 

Klebs*  studied  certain  algae  and  fungi  in  order  to  de- 
termine the  conditions  under  which  they  reproduce  them- 
selves. By  experiment  in  culture  he  found  that  when  cer- 
tain conditions  prevail,  Vaucheria  will  form  zoospores, 
under  other  influences  it  will  develop  sexual  organs  and 
reproduce  itself  through  them.  These  conditions  are  repre- 
sented in  tabular  form  thus : 

VAUCHERIA  SESSILIS  f 

ZOOSPORES  SEXUAL  ORGANS 

Darkness  after  sufficient  illumina-  Light  absolutely  necessary  for  the 
tion  for  adequate  food-manufacture  formation  of  sex-organs.  Light  suf- 
invariably  induces  zoospore  forma-  ficient  for  healthy  growth  may  be 
tion.  This  will  continue  in  darkness  insufficient  for  formation  of  sex- 
till  there  is  no  longer  enough  food,  organs.  Strong  light  needed, 
though  there  may  still  be  enough  Light,  apart  from  its  effect  on 
for  slow  growth.  nutrition,  is  a  direct  stimulus  to 

form  sex-organs. 

The  addition  of  water  to  a  culture  Sex-organs  form  either  in  damp 
in  damp  air  induces  active  zoospore  air  or  in  water ;  better  in  the  for- 
formation.  mer. 

Temperature  of  3°  C.  inhibits  zoo-         Same  for  sex-organs, 
spore  formation  in    all    but   accli- 
mated forms. 

*  Klebs,  G.  Die  Bedingungen  der  Fortpflanzung  bei  einigen  Algen  und 
Pilzen.  Jena.  1896.  The  literature  is  here  cited.  A  second  volume,  treat- 
ing of  the  general  questions  of  the  physiology  of  reproduction  in  low  or- 
ganisms, is  promised,  but  not  yet  published  (1902).  See  also  Jahrb.  f. 
wiss.  Bot.,  Bd.  32, 33, 35.  Falck,  R.  Die  Bedingungen  und  die  Bedeutung  der 
Zygotenbildung  bei  Sporodinia  grandis.  Beitrage  z.  Biol.  d.  Pflanzen,  Bd. 
VIII,  1901. 

f  Klebs  divides  this  species  into  three.  The  names  of  these,  and  the 
reasons  for  division,  not  being  essential  in  this  connection,  I  retain  the 
old  name. 


REPRODUCTION 


265 


ZOOSPORES 

3°-8  are  constant  stimulus  to 
zoospore  formation. 

26 '  is  maximum  temperature  for 
zoospore  formation  but  not  for 
growth.  No  acclimation  to  this. 

10~-20r  are  indifferent  tempera- 
tures, exercising  neither  stimulating 
nor  depressing  effects  in  passing 
up  or  down. 

Raising  temperature  from  indiffer- 
ent to  high,  or  lowering  from  in- 
different to  low,  exercises  no  stimu- 
lus; but  raising  from  3°  to  15° 
stimulates. 

Culture  in  inorganic  nutrient  solu- 
tion favors  nutrition  and  growth, 
but  not  zoospore  formation.  Trans- 
fer from  such  to  less  nutritious  acts 
as  stimulus  to  zoospore  formation. 

Sugars  and  other  nutritious  or- 
ganic compounds  do  not  stimulate 
to  formation  of  zoospores. 

More  oxygen  required  than  for 
growth. 

Flowing  water,  by  favoring  nutri- 
tion and  growth,  retards  or  pre- 
vents zoospore  formation.  Con- 
versely, transfer  to  still  water  acts 
as  a  stimulus.  Accommodation  to 
still  water  followed  by  rapid  growth 
and  cessation  of  zoospore  forma- 
tion. 

From  this  table  certain  influences  stand  out  as  especially 
efficient  stimuli  to  the  formation  of  reproductive  bodies. 
Thus  light,  which  furnishes  the  energy  for  the  manufacture 
of  the  materials  needed  to  form  the  sexual  organs  and  ele- 
ments, must  fall  upon  the  plant  in  intensity,  not  only 
sufficient  for  this,  but  also  great  enough  to  act  as  a  distinct 
stimulus  to  use  the  manufactured  substance  in  this  special 
way.  Light  furnishes  the  energy  for  one  kind  of  work ;  more 
energy  of  this  same  kind  sets  and  keeps  other  processes  in 
operation.  Food,  when  available  in  abundance,  thus  reliev- 
ing the  plant  of  the  necessity  of  diligently  manufacturing 


SEXUAL  ORGANS 

Sex-organs  form,  but  develop  more 
slowly  than  at  higher  temperatures. 
Same  for  sex-organs. 


Same  for  sex-organs. 


Same  for  sex-organs. 


Same    for    sex-organs. 


Favor  formation  and  develop- 
ment of  sex-organs. 

More  oxygen  required  than  for 
zoospore  formation. 

No  fertile  plants  in  running  water. 
Become  fertile  in  still  water. 


266  PLANT  PHYSIOLOGY 

food  for  itself,  stimulates  to  sexual  activity,  as  is  shown  by 
the  formation  and  activity  of  the  sex-organs.  This  seems 
to  be  universally  true  in  nature.  Given  the  need  of  food 
and  the  means  of  manufacturing  or  obtaining  it,  the  plant 
will  be  so  especially  engaged  in  the  processes  of  nutrition 
that  reproduction  is  not  undertaken.  But  on  the  other 
hand,  given  a  sufficient  supply  of  food  stored  in  its  own 
body  and  diminished  or  suspended  means  of  manufacturing 
more  food,  the  plant  will  produce  for  a  time  in  still  water 
both  zoospores  and  sexual  elements,  while  in  the  dark  the 
formation  of  zoospores  (not  of  sex-organs)  will  continue 
for  some  length  of  time. 

If  plants  are  healthy,  certain  conditions  will  invariably 
induce  them,  whether  they  are  old  or  young,  to  remain 
sterile.  Certain  other  conditions  will  induce  them  to  form 
bodies  for  their  non-sexual  reproduction,  still  other  condi- 
tions will  induce  them  to  reproduce  sexually.  Reproduction 
then  must  be  regarded,  like  movement,  as  a  reaction  or 
response  to  stimuli.  But  just  as  stimuli  the  same  in  kind 
and  degree  induce  one  response  or  the  opposite  or  none  at 
all,  according  to  the  kind  of  organism,  so  in  reproduction, 
a  stimulus  or  a  combination  of  stimuli  which  induces  one 
kind  of  plant  to  reproduce  itself  sexually  may  induce  other 
kinds  to  reproduce  themselves  non-sexually  or  not  at  all. 
This  may  b^  illustrated  by  Klebs's  studies  of  Hydrodictyon. 
This  plant,  the  water-net,  lives  in  still  or  only  slowly  run- 
ning water,  under  normal  conditions  floats,  and  is  physio- 
logically as  well  as  mechanically  a  very  sensitive  organism. 
It  possesses  two  very  distinct  methods  of  reproduction,  by 
non-sexual  zoospores,  and  by  sexual  motile  spores  or 
gametes.  Gametes  and  zoospores  are  formed  in  the  ordi- 
nary vegetative  cells,  not  in  specially  differentiated  organs. 
Until  these  cells  have  attained  a  certain,  though  very 
minute,  size  they  cannot  form  reproductive  bodies.  Whether 
there  is  a  maximum  size  or  not  cannot  be  positively  stated. 
Since  gametes  and  zoospores  form  in  vegetative  cells  indis- 
tinguishable from  each  other,  and  since  all  or  some  of  the 
cells  of  a  net  will  form  zoospores  or  gametes  or  neither,  it  is 
obvious  that  each  cell  possesses  in  equal  measure  the  ability 


* 


REPRODUCTION 


267 


to  reproduce  itself  by  zoospores  or  by  gametes.  Observa- 
tion shows  fchat  the  same  cell  never  reproduces  itself  by  both 
means.  External  influences  must,  therefore,  swing  the 
balance  hi  one  direction  or  the  other. 

The   following  is  a  table  showing  the  results  of  Klebs's 
experiments  on  Hydrodictyon : 

HYDRODICTYON  UTRICULATUM 

ZOOSPORES  GAMETES 


Light  absolutely  necessary  to  zoo- 
spore  formation,  strong  light  most 
favorable. 

Darkness  checks  zoospore  forma- 
tion. 

Transfer  from  darkness  to  light 
stimulates. 

Flowing  water  retards  or  pre- 
vents zoospore  formation,  still 
water  stimulates,  considerable 
volume  needed. 

Inorganic  nutrient  salts  very 
strongly  induce  zoospores  to  form. 
Zoospores  especially  abundant 
when  transfer  in  the  light  from  nu- 
trient solution  to  water. 

Cane  sugar  not  stimulating. 
Maltose  very  stimulating  in  light. 


32°     maximum   temperature    for 
zoospores. 

8°  too  low  temperature  for  zoo- 
spores. 


Only  floating  nets  form  zoospores. 


Light  not  absolutely  necessary. 


Darkness  favors  gamete  over  zoo- 
spore  formation. 

Transfer  from  light  to  darkness 
stimulates. 

Small  volume  of  still  water  stimu- 
lates gametes  to  form. 


Inorganic    nutrient 
gamete  formation. 


salts   retard 


Cane  sugar  in  O.Sjg-lO^  stimulates. 

Maltose  and  cane  sugar  stimulate 
in  darkness. 

Cells  with  slight  tendency  to  zoo- 
spore  formation  produce  zoospores 
in  lighted,  gametes  in  darkened 
maltose  solution. 

33r-34  maximum  temperature  for 
gametes. 

Same  for  gametes. 

Slightly  higher  temperature  will 
retard  zoospore  and  stimulate  ga- 
mete formation. 

Nets  grown  on  filter  paper  wet 
with  water,  or  better,  with  cane 
sugar  solution,  form  gametes. 


268  PLANT  PHYSIOLOGY 

This  table  shows  that  those  conditions  which  contribute 
to  increase  and  to  maintain  those  activities  which  supply 
the  plant  with  food,  also  favor  reproduction  by  non-sexual 
rneans.  Light,  enabling  the  plant  to  manufacture  carbohy- 
drates, and  the  salts  needed  in  elaborating  carbohydrates 
into  amides  and  proteids,  stimulate  zoospore  formation.  On 
the  other  hand,  the  copious  supply  of  already  elaborated 
carbohydrates,  especially  when  no  unusual  amount  of  in- 
organic salts  stimulates  to  the  further  elaboration  of  carbo- 
hydrates into  amides  and  proteids,  is  most  favorable  to  the 
formation  of  gametes.  Light  is  a  specific  stimulus  to  zoo- 
spore  formation,  apart  from  its  influence  on  nutrition. 
Given  a  vegetating  cell,  healthy  and  of  such  size  that  it  can 
divide  into  zoospores  or  gametes,  it  will  continue  to  vege- 
tate until  it  is  subjected  to  some  influence  which  will  cause 
it  to  grow  more  slowly  or  to  cease  growing,  and  which  will 
stimulate  it  to  reproduce  itself  in  the  one  or  the  other  of  the 
two  ways  in  which  it  can  reproduce.  Hydrodictyon  shows 
plainly  that  the  time  and  the  manner  of  reproduction  are 
not  fixed,  but  that  both  are  determined  by  the  influences  to 
which  the  plant  is  subjected  and  is  sensitive.  Additional 
evidence  of  this  is  furnished  by  Klebs's  observation  that 
Hydrodictyon  nets  with  an  already  marked  tendency  to 
form  zoospores  can  be  so  influenced  by  external  conditions 
that  they  will  cease  to  form  zoospores  and  will  thereupon 
produce  gametes  instead.  The  same  net,  darkened  at  one 
end  and  illuminated  at  the  other,  will  form  gametes  in  the 
one  end,  zoospores  in  the  other,  respectively. 

Livingston*  has  studied  one  of  the  polymorphic  algae,  a 
StigeoclonJum,  particularly  with  a  view  to  determining  the 
influence  of  varying  concentrations  of  the  medium  upon  the 
form  and  reproduction  of  the  plants.  He  finds  that  de- 
creased osmotic  pressure  acts  as  a  stimulus  to  zoospore- 
formation  as  well  as  favoring  vegetative  activities,  and  that 
high  osmotic  pressure  checks  zoSspore-formation  and  the 

*  Livingston.  B.  E.  Nature  of  the  stimulus  which  causes  change  of 
form  in  polymorphic  algre.  Bot.  Gazette  vol.  30.  1900.  Further  notes 
on  the  physiology  of  polymorphism  in  green  algae.  Bot.  Gazette,  vol.  32, 
1901. 


REPRODUCTION  269 

vegetative  activities.  This  corresponds  with  Klebs's  obser- 
vations, but  puts  one  part  of  them  in  somewhat  more  defi- 
nite terms. 

We  see,  then,  that  so  far  as  the  fresh-water  algae  and  • 
certain  fungi  are  concerned,  plants  react  in  reproduction,  as 
in  other  ways,  to  external  influences.  In  regions  where 
there  are  clearly  marked  seasons,  during  one  of  which  active 
vegetation  is  impossible  by  reason  of  extreme  cold  or  dark- 
ness or  dryness,  these  violent  conditions  determine  the 
behavior  of  the  organisms  living  there.  In  milder  seasons 
and  in  more  constantly  temperate  regions,  more  moderate 
influences  determine  their  behavior.  Of  the  forces  which  act 
upon  algap,  stimulating  processes  which  can  be  accom- 
plished without  them,  light  and  heat  are  evidently  the  most 
important.  A  certain  minimum  amount  of  heat  is  a  neces- 
sary condition  of  life;  without  it  action  is  impossible;  but 
given  this  minimum  amount,  more  heat  will  stimulate  to 
reproduction.  A  certain  minimum  amount  of  light  is  the 
necessary  source  of  the  energy  for  food-manufacture,  but 
given  this  amount,  more  light  will  stimulate  to  some  form 
of  reproduction.  Neither  this  larger  amount  of  external 
heat  nor  the  more  intense  light  is  necessary  as  a  means  of 
carrying  on  reproduction;  they  only  set  in  operation  that 
succession  of  processes  which  terminates  in  the  formation  of 
new  individuals.  Among  the  fungi,  Klebs*  claims  that 
changes  in  the  nutrition  furnish  the  stimulus  to  a  well- 
nourished  mycelium  to  develop  reproductive  organs. 

Vochting's  work  on  the  conditions  of  reproduction  in 
flowering  plants  t  yielded  results  with  which  those  of  Klebs 
harmonize.  Flowers  are  the  visible  agents  of  sexual  repro- 
duction in  higher  plants.  When  they  are  suppressed,  or  even 
are  imperfect,  sexual  reproduction  does  not  take  place. 
Sexual  reproduction  in  higher  plants  consists,  as  in  lower 
forms,  in  the  union  of  the  microscopically  small  sexual 
elements.  The  forces  which  institute,  stimulate,  and  favor 

*  Klebs  G.  Zur  Physiologic  der  Fortpflanzung  einiger  Pilze.  Jahrb.  f. 
wiss.  Bot..  pp.  146-7,  Bd.  35.  1900. 

\  Vochting.  H.  f  ber  den  Einfluss  des  Lichtes  auf  die  Gestaltung  und 
Anlage  der  Bliithen.  Jahrb.  f.  wiss.  Bot..  Bd.  25,  1893. 


270  PLANT  PHYSIOLOGY 


sexual  reproduction  can,  therefore,  be  ascertained  by  study- 
ing those  forces  which  influence  the  conspicuous  parts,  the 
flowers.  A  force  which  must  attain  a  certain  strength  before 
it  will  stimulate  the  plant  to  blossom,  and  without  which 
the  plant  blossoms  but  imperfectly  if  at  all,  is  one  which 
controls  sexual  reproduction.  Without  this  force  the  plant 
may  reproduce  itself  by  non-sexual  means,  by  vegetative 
processes  such  as  the  formation  of  runners,  suckers,  etc. 
Under  the  influence  of  this  force  it  will  multiply  only  by 
sexual  means.  In  such  a  case,  the  one  force  would  control 
reproduction,  both  sexual  and  non-sexual.  The  effect  of  the 
force  as  a  controlling  agent  is  due  to  its  own  direct  influ- 
ence upon  the  plant  and  also  to  the  irritable  response  of  the 
plant  to  its  influence,  the  response  setting  in  motion  other 
forces  in  its  own  body.  The  force  from  without  is  but  the 
initial  energy  which  releases  other  quantities  of  energy,  thus 
setting  in  operation  other  processes  than  those  which  it 
can  directly  affect. 

Of  the  many  plants  studied  by  Vochting,   Mimulus   Ti- 
lingi  serves  best  to  illustrate  the  facts  which  he  has  brought 
out.    Figures  22  and  23  in  the  accompanying  illustration 
indicate  the  normal  habit  of  potted  plants  of  this  species, 
respectively    a  blooming  and  vigorous  plant  from  a  last 
year's  cutting,   and  a  much  younger  one   not   yet  ready 
to  bloom.    The  vegetative  functions,  as  well  as  the  vegeta- 
tive mode  of  reproduction,  of  the  plant,  are  carried  out  by 
the  usually  more  or  less  creeping  leafy  branches  originating 
on  the  stem  just  above  the  surface  of  the  soil.    When  the 
vegetating   period    draws   toward    the   close,    the  new  in- 
ternodes  of  these  branches  are  successively  shorter,  the  tips 
of  the  branches  forming  finally  rosette-like  bunches  of  leaves 
on  the  surface  of  the  soil.     Such  a  bunch  cut  off  and  trans- 
planted   will  presently    give    rise    to    the    flowering  erect 
branch  shown  in  figure  22.    The  rosettes  will  form  erect 
branches,    however,   only  when  warm  enough.    The  upper 
part  of  an  erect  branch  bears  flowers.    Just  below  these  are 
a  few  pairs  of  leaves  in  the  axils  of  which  short  vegetative 
branches  begin  to  form.    The  flowering  part  of  the  branch 
may  bear  lateral  flowering  branchlets  if  the  plant  is  vig- 


REPRODUCTION 


271 


orous.  After  fruiting,  the  whole  branch  usually  dies  to  the 
ground,  and  then  the  rosette  from  which  it  arose  will  give 
rise  to  the  lateral  creeping  branches  by  which  new  rosettes 
will  be  formed.  Thus  non-sexual  vegeta- 
tive reproduction  follows  the  sexual  mode, 
the  same  plant  being  capable  of  both  but 
not  of  both  simultaneously.  Before  it  has 
flowered,  however,  a  rosette  is  not  likely 
soon  to  branch  and  to  form  new  rosettes. 
If  vigorous  young  plants  which,  under 
ordinary  conditions,  would  presently  bloom 
are  so  placed  that  they  have  only  enough 
light  for  active  vegetation  but  otherwise 
are  very  favorably  situated,  they  will  not 
form  the  erect  flower-bearing  branches,  but 
will  continue  to  grow  and  will  presently 
form  creeping  branches,  spreading  and 
maintaining  themselves  in  this  way.  Plants 
may  be  kept  from  blooming  by  this  means 


Fig.  23  Fig.  22 

Figures  22.  23.  Mimulus  Tiling}.  Fignre  22.  a  blooming  plant  from 
a  cutting  of  the  preceding  year.  Figure  23,  a  younger  plant  not  yet 
ready  to  bloom.  (After  Vochting). 

for  an  indefinite  length  of  time.  Vochting  reports  having 
kept  plants  sterile  for  three  years.  The  effect  of  such 
enforced  sterility  on  the  health  of  the  plants  appears 
to  be  neither  favorable  nor  unfavorable.  Under  other- 
wise like  favorable  conditions  plants  which  have  not  flow- 
ered present  as  vigorous  an  appearance  as  those  which 


272  PLANT  PHYSIOLOGY 

have  flowered  and  seeded.  In  every  way  they  behave  alike. 
It  would  seem  then  that  these  plants  require,  as  the  stim- 
ulus to  form  flowering  branches,  an  intensity  of  light  which 
is  more  than  sufficient  to  maintain  the  vegetative  activi- 
ties at  a  perfectly  healthy  pitch. 

Vochting  further  shows  that  various  degrees  of  illumina- 
tion above  what  is  sufficient  for  the  vegetative  processes, 
but  insufficient  for  those  connected  with  sexual  reproduc- 
tion, will  produce  effects  exactly  corresponding  to  the  de- 
gree of  illumination.  Light  enough  to  induce  the  formation 
of  an  erect  branch  and  its  growth  to  a  height  of  several 
centimetres  may  be  insufficient  to  induce  the  formation  of 
any  flower-buds  upon  it.  Still  stronger  illumination  will  in- 
sure the  formation  of  flower-buds  and  their  attaining  a  very 
considerable  size,  but  will  not  induce  them  to  open.  The 
bearing  of  this  fact  on  the  formation  of  cleistogamous 
flowers  Yochting  considers  very  important.  Such  a  degree 
of  illumination,  instead  of  inducing  the  perfect  development 
and  opening  of  flower-buds,  will  stimulate  the  vegetative 
buds  in  the  axils  of  the  pairs  of  leaves,  just  below  the 
flowering  part  of  the  erect  branch,  to  develop.  Still  stronger 
light,  inducing  the  opening  of  the  flower-buds,  if  still  in- 
sufficient, is  indicated  by  the  small  size  of  the  flowers  or 
by  the  rudimentary  condition  of  some  of  the  floral  organs. 
The  corolla,  and  in  two-lipped  flowers  the  upper  lip,  give 
evidence  by  their  imperfect  development  of  slightly  in- 
sufficient light,  while  to  suppress  or  to  reduce  the  cal^yx 
still  less  light  must  fall  upon  the  plant.  The  essential 
organs,  stamens  and  pistils,  are  least  dependent  upon  light. 
However,  the  flower  as  a  whole,  all  of  its  parts,  and  even 
the  branch  which  bears  it,  are  dependent  for  their  forma- 
tion upon  illumination  of  sufficient  intensity  and  duration. 

The  light  evidently  acts  as  a  stimulus,  inducing  certain 
effects,  for  if  the  formation  of  a  flowering  branch  has  be- 
gun, its  growth  will  continue  for  a  time  even  in  darkness. 
If  the  formation  of  flowers  on  the  branch  has  begun,  they 
too  will  continue  to  grow  in  darkness.  But  to  insure  the 
perfect  development  of  the  flowers  of  most  plants,  suffi- 
ciently intense  illumination,  repeated  with  sufficient  fre- 


REPRODUCTION  273 

quency,  is  absolutely  necessary.  In  other  words,  the  stim- 
ulus is  but  transient.  Repeated  stimulation  is  needed  to 
continue  in  operation  the  processes  by  which  the  formation, 
growth,  and  perfect  development  of  flowers,  and  their  ac- 
cessory parts,  are  accomplished.  In  some  few  plants,  flowers 
which  have  been  duly  induced  to  form  will  develop  per- 
fectly in  the  dark,  but  in  most  plants  abnormal  characters 
will  appear,  varying  in  importance  from  an  insufficiency  of 
color  to  deformity  or  suppression  of  parts,  or  the  failure  of 
the  flowers  to  open. 

Sachs's  hypothesis,  *  that  certain  specific  substances,  pre- 
sumably made  in  the  leaves  under  the  influence  of  light,  are 
necessary  for  the  formation  of  flowers,  breaks  down  under 
Vochting's  experiments,  for  we  cannot  reasonably  admit  the 
presence  in  the  plant  of  corolla-forming  material  as  distinct 
from  calyx-forming,  etc. 

When  a  plant  is  so  well  nourished,  has  attained  such  a 
size,  and  is  under  such  favorable  conditions  generally,  that 
it  is  able  to  form  the  organs  and  substances  of  sexual  re- 
production, it  must  be  that  it  will  do  so  whenever  the  proc- 
esses concerned  in  sexual  reproduction  are  set  and  main- 
tained in  operation  by  a  stimulus  from  outside.  This 
stimulus  is  the  light.  Without  it,  the  higher  plant  cannot 
reproduce,  or  even  prepare  to  reproduce,  itself  sexually. 
The  plant  is  sensitive  to  that  force  which,  in  cases  where  the 
plant  is  dependent  upon  insects  for  cross-pollination,  will  in 
so  far  as  possible  ensure  the  visits  of  the  necessary  insects. 
It  would  be  interesting  to  determine  whether  other  than 
entomophilous  flowers  are  so  sensitive  to  light  and  so 
dependent  upon  it. 

The  influence  of  light  is  to  be  distinguished  from  the  in- 
fluences of  food,  water,  and  of  the  other  substances,  of  heat 
arid  of  the  other  forces,  essential  to  active  life.  With  in- 
sufficient warmth,  food,  or  water,  the  plant  will  be  imper- 
fectly able,  or  quite  unable,  to  reproduce  itself  by  sexual 
means ;  but  this  inability  is  due  to  the  effect  of  unfavorable 
conditions  upon  all  of  its  vital  functions.  Warmth,  food, 

*  Sachs.  J.  von.  Lectures  on  the  physiology  of  plants.  English  Ed., 
Oxford  1887.  and  his  special  papers  therein  referred  to  (pp.  530.  534.  etc.) 

18 


276  PLANT  PHYSIOLOGY 

to  change  their  behavior  accordingly.  So,  also,  changes 
of  other  sorts,  making  the  conditions  suitable  not  only  for 
the  growth  and  healthy  life  of  the  individual  but  also  for 
the  production  and  active  life  of  new  individuals,  will  induce 
plants  to  form  these  by  sexual  or  by  non-sexual  means, 
according  to  the  species  of  plant.  In  the  spring,  when  the 
conditions  for  active  vegetation  are  becoming  more  and 
more  favorable,  the  Equisetums  complete  the  development 
of  the  spores  non-sexually  formed  in  the  foregoing  season, 
and  shed  these  spores,  which  germinate  at  once,  if  at  all, 
and  form  new  plants,  the  prothalli.  These,  if  conditions 
favor,  soon  develop  sexual  organs  and  produce  new  plants 
by  sexual  means.  In  these  alternating  generations,  the  non- 
sexual  and  the  sexual,  which  follow  one  another  in  suc- 
cessive years,  the  conditions  of  the  later  growing  period 
of  one  year  induce  the  mature  plants  to  form  spores  by 
non-sexual  means.  The  different  conditions  of  the  earliest 
growing  period  of  the  following  year  induce  the  same  plants 
to  complete  the  development  of  the  spores  already  formed. 
The  still  different  conditions  of  the  days  and  weeks  suc- 
ceeding induce  these  spores  to  germinate,  the  prothalli  to 
form  archegonia  and  antheridia,  the  embryos  to  develop 
into  the  larger  non-sexual  plants.  These  last  vegetate 
for  an  indefinite  length  of  time.  The  successive  changes  in 
the  seasons,  in  the  soil,  and  in  the  water-supply,  fix  the 
succession  of  generations  which  recur  according  to  the 
peculiar  adjustment  of  the  species  to  all  the  factors  of  its 
environment.  Species,  genera,  families,  and  orders  differ  in 
their  adjustments  and  in  their  reactions  to  the  stimuli  given 
by  the  many  different  factors  composing  the  environment. 
Comparing  sexual  and  non-sexual  reproduction,  we  see 
that  the  advantage  of  the  parents  determines  what  mode 
of  reproduction,  if  any,  shall  be  carried  out.  For  a  long  time 
it  has  been  supposed  that  sexual  reproduction  is  distinctly 
advantageous  to  the  species,  if  not  occasionally  altogether 
necessary  to  those  forms  capable  of  reproducing  themselves 
by  this  means.  Such  a  view  is  not  quite  correct.  Many 
species  of  plants,  among  them  forms  which  are  eminently 
successful  as  judged  by  their  numbers  and  activity,  are 


REPRODUCTION  277 

wholly  incapable  of  reproducing  themselves  by  sexual  means. 
The  bacteria,  the  higher  fungi,  and  many  independent 
plants  illustrate  this.  Some  of  the  higher  algae,  archegoni- 
ates,  and  flowering  plants  may  go  on  indefinitely,  without 
reproducing  themselves  by  other  than  purely  vegetative 
means,  and  with  no  evidence  of  injury  to  the  individual  or 
to  the  species.  It  is  only  when  special  conditions  produce 
special  effects  on  plants,  that  they  need  to  reproduce  them- 
selves by  sexual  or  non-sexual  spores.  Under  special  con- 
ditions, reproduction  becomes  a  necessity  by  becoming 
advantageous  to  the  individual.  The  species  profits  ac- 
cordingly. 

Certain  advantages  to  the  species  are  conceivable  in  the 
spore  method  of  reproduction  over  any  vegetative  method, 
and  certain  advantages  are  conceivable  in  the  sexually 
produced  offspring  over  those  non-sexually  produced,  but 
these  are  conceptions  rather  than  proved  facts  in  all  but  a 
very  few  cases.  These  few  cases  may  or  may  not  be  typical 
of  the  majority.  It  is  conceivable,  for  example,  that  the 
formation  of  new  individuals  by  runners,  as  in  the  straw- 
berry, and  by  suckers,  as  in  the  coast  red- wood  (Sequoia 
.semper virens ),  is  advantageous  to  the  species.  The  new 
individuals  hold  the  territory  won  by  their  parents  and 
may  even  extend  it  somewhat.  The  young  are  nourished 
by  the  parent  until  they  have  attained  size,  strength,  and 
development  which  give  them  an  advantage  in  competing 
with  other  plants  for  space  and  for  the  means  of  existence. 
These  means  of  reproduction  are,  however,  defective  in 
that  they  do  not  secure  the  dispersal  of  the  new  individuals 
and  consequently  do  not  tend  to  insure  the  survival  of 
the  species  or  to  extend  its  territory  rapidly  enough.  The 
survival  of  the  well-equipped  offspring  formed  by  vegetative 
means  is  certain  so  long  as  the  conditions  in  the  space 
occupied  by  the  parent  remain  favorable,  but  such  repro- 
duction is  dangerous,  because  wide  dispersal  will  secure  for 
the  species  some  positions  at  least  in  which  it  can  be  main- 
tained whatever  may  befall  individuals  in  other  places. 
A  puddle  may  become  densely  populated  through  the  vege- 
tative reproduction  of  the  alga?  and  animals  started  there- 


276  PLANT  PHYSIOLOGY 

to  change  their  behavior  accordingly.  So,  also,  changes 
of  other  sorts,  making  the  conditions  suitable  not  only  for 
the  growth  and  healthy  life  of  the  individual  but  also  for 
the  production  and  active  life  of  new  individuals,  will  induce 
plants  to  form  these  by  sexual  or  by  non-sexual  means, 
according  to  the  species  of  plant.  In  the  spring,  when  the 
conditions  for  active  vegetation  are  becoming  more  and 
more  favorable,  the  Equisetums  complete  the  development 
of  the  spores  non-sexually  formed  in  the  foregoing  season, 
and  shed  these  spores,  which  germinate  at  once,  if  at  all, 
and  form  new  plants,  the  prothalli.  These,  if  conditions 
favor,  soon  develop  sexual  organs  and  produce  new  plants 
by  sexual  means.  In  these  alternating  generations,  the  non- 
sexual  and  the  sexual,  which  follow  one  another  in  suc- 
cessive years,  the  conditions  of  the  later  growing  period 
of  one  year  induce  the  mature  plants  to  form  spores  by 
non-sexual  means.  The  different  conditions  of  the  earliest 
growing  period  of  the  following  year  induce  the  same  plants 
to  complete  the  development  of  the  spores  already  formed. 
The  still  different  conditions  of  the  days  and  weeks  suc- 
ceeding induce  these  spores  to  germinate,  the  prothalli  to 
form  archegonia  and  antheridia,  the  embryos  to  develop 
into  the  larger  non-sexual  plants.  These  last  vegetate 
for  an  indefinite  length  of  time.  The  successive  changes  in 
the  seasons,  in  the  soil,  and  in  the  water-supply,  fix  the 
succession  of  generations  which  recur  according  to  the 
peculiar  adjustment  of  the  species  to  all  the  factors  of  its 
environment.  Species,  genera,  families,  and  orders  differ  in 
their  adjustments  and  in  their  reactions  to  the  stimuli  given 
by  the  many  different  factors  composing  the  environment. 
Comparing  sexual  and  non-sexual  reproduction,  we  see 
that  the  advantage  of  the  parents  determines  what  mode 
of  reproduction,  if  any,  shall  be  carried  out.  For  a  long  time 
it  has  been  supposed  that  sexual  reproduction  is  distinctly 
advantageous  to  the  species,  if  not  occasionally  altogether 
necessary  to  those  forms  capable  of  reproducing  themselves 
by  this  means.  Such  a  view  is  not  quite  correct.  Many 
species  of  plants,  among  them  forms  which  are  eminently 
successful  as  judged  by  their  numbers  and  activity,  are 


REPRODUCTION  277 

wholly  incapable  of  reproducing  themselves  by  sexual  means. 
The  bacteria,  the  higher  fungi,  and  many  independent 
plants  illustrate  this.  Some  of  the  higher  algae,  archegoni- 
ates,  and  flowering  plants  may  go  on  indefinitely,  without 
reproducing  themselves  by  other  than  purely  vegetative 
means,  and  with  no  evidence  of  injury  to  the  individual  or 
to  the  species.  It  is  only  when  special  conditions  produce 
special  effects  on  plants,  that  they  need  to  reproduce  them- 
selves by  sexual  or  non-sexual  spores.  Under  special  con- 
ditions, reproduction  becomes  a  necessity  by  becoming 
advantageous  to  the  individual.  The  species  profits  ac- 
cordingly. 

Certain  advantages  to  the  species  are  conceivable  in  the 
spore  method  of  reproduction  over  any  vegetative  method, 
and  certain  advantages  are  conceivable  in  the  sexually 
produced  offspring  over  those  non-sexually  produced,  but 
these  are  conceptions  rather  than  proved  facts  in  all  but  a 
very  few  cases.  These  few  cases  may  or  may  not  be  typical 
of  the  majority.  It  is  conceivable,  for  example,  that  the 
formation  of  new  individuals  by  runners,  as  in  the  straw- 
berry, and  by  suckers,  as  in  the  coast  red-wood  (Sequoia 
.semper virens),  is  advantageous  to  the  species.  The  new 
individuals  hold  the  territory  won  by  their  parents  and 
may  even  extend  it  somewhat.  The  young  are  nourished 
by  the  parent  until  they  have  attained  size,  strength,  and 
development  which  give  them  an  advantage  in  competing 
with  other  plants  for  space  and  for  the  means  of  existence. 
These  means  of  reproduction  are,  however,  defective  in 
that  they  do  not  secure  the  dispersal  of  the  new  individuals 
and  consequently  do  not  tend  to  insure  the  survival  of 
the  species  or  to  extend  its  territory  rapidly  enough.  The 
survival  of  the  well-equipped  offspring  formed  by  vegetative 
means  is  certain  so  long  as  the  conditions  in  the  space 
occupied  by  the  parent  remain  favorable,  but  such  repro- 
duction is  dangerous,  because  wide  dispersal  will  secure  for 
the  species  some  positions  at  least  in  which  it  can  be  main- 
tained whatever  may  befall  individuals  in  other  places. 
A  puddle  may  become  densely  populated  through  the  vege- 
tative reproduction  of  the  algae  and  animals  started  there- 


278  PLANT  PHYSIOLOGY 

in,  but  unless  these  give  rise  to  spore  or  other  resting 
stages  all  the  species  will  be  locally  destroyed  when  the 
puddle  dries.  Vegetative  reproduction  is  in  many  cases 
the  most  effective  means  of  multiplying  the  number  of  in- 
dividuals, but  it  is  seldom  effective  in  securing  wide  dis- 
persal or  in  insuring  the  permanence  of  the  species.  These 
two  needs  are  generally  met  by  the  formation  of  non- 
sexual  or  sexual  spores  and  their  products  (seeds,  fruits). 
The  non-sexually  formed  spores  of  the  ferns,  and  arche- 
goniates  generally,  and  of  the  lower  plants,  fungi  and  alga?, 
are  especially  adapted  by  their  small  size  and  in  various 
special  ways  to  secure  wide  dispersal.  They  may  or  may 
not  be  very  resistant.  According  to  the  plant  which  forms 
them,  and  according 'to  the  season  at  which  they  are  formed, 
they  will  be  resting-spores  or  such  as  must  germinate  at 
once  if  at  all. 

New  individuals  vegetatively  produced,  and  those  from 
germinating  non-sexual  spores,  will  contain  material  from 
their  single  parent  only.  By  sexual  means,  there  are  united 
in  the  offspring  substances  from  both  parents.  Among  the 
lowest  plants  ( and  animals )  the  visible  differences  between 
the  two  parents  are  only  slight.  In  higher  plants,  however, 
where  the  new  individuals  are  formed  by  the  fusion  of 
sexual  elements  themselves  obviously  unlike,  formed  in 
organs  and  by  parents  also  obviously  unlike,  it  is  evident 
that  a  new  balance  of  forces  and  matters  may  be,  but  not 
necessarily  will  be,  possessed  by  the  offspring.  Maternal 
and  paternal  characters  may  offset  or  intensify  one  another ; 
maternal  influences  during  the  development  of  the  spore  or 
embryo  may  or  may  not  neutralize  the  paternal  influence 
carried  into  the  germ-cell  by  the  sperm.  The  offspring 
sexually  produced,  representing  a  new  adjustment,  may  be 
better  adapted  to  prevailing  conditions  than  was  either 
parent.  These  are  all  possibilities,  seldom  probabilities, 
almost  never  certainties.  The  chances  are  even  that  the 
offspring  will  be  like  the  average  of  the  species.  Some  may 
be  worse,  a  few  may  be  better,  but  unless  the  better,  when 
it  comes  their  turn  to  reproduce  sexually,  mate  with  others 
equally  good,  their  offspring  will  almost  certainly  be  like  the 


REPRODUCTION  279 

average  of  the  species.  The  statistical  study  of  variation,* 
now  so  much  the  fashion  among  biologists,  shows  how  wide 
the  range  of  variation  is.  The  inevitable  result  of  combin- 
ing two  different  things  is  the  production  of  a  mean  or 
balance.  In  this  balancing  or  equalizing  ( Ausgleich ) ,  Stras- 
burgerf  sees  the  essential  character  of  sexual  reproduction 
and  its  fundamental  advantage  over  all  other  modes  of 
reproduction. 

In  certain  species  sexual  reproduction  seems  to  be  abso- 
lutely necessary  to  escape  degeneration  and  extinction. 
This  has  been  shown  to  be  the  case  among  certain  Infusoria 
and  Diatoms,  but  these  are  special  cases  in  which  the  indi- 
viduals by  prolonged  division  (vegetative  reproduction), 
incompletely  offset  by  growth,  have  become  unduly  small 
and  weak.  By  the  fusion  of  two  such  individuals  the  nor- 
mal amount  of  substance  and  the  normal  means  of  secur- 
ing energy  are  furnished  to  the  new  individuals.  Except 
in  these  special  cases,  and  in  the  highest  animals,  sexual 
reproduction  does  not  seem  to  be  necessary  to  the  mainte- 
nance of  the  species  in  a  healthy  condition,  in  which  it 
stands  a  good  chance  to  succeed  in  the  struggle  for  ex- 
istence. 

HEREDITY 

The  physiologist  is  confronted  with  the  problem  of  hered- 
ity. He  sees  plants  resembling  one  another  in  succeeding 
generations  and  he  hears  discussions  of  the  means  by  which 
the  characters  and  the  tendencies  of  the  parents  are  trans- 
mitted to  the  offspring.  What  is  "heredity,"  and  what  are 
the  means  by  which  the  offspring  are  made  to  possess  the 
characters  and  tendencies  of  their  parents?  "Heredity  is  the 
biological  law  by  which  living  beings  tend  to  repeat  their 
characteristics  in  their  descendants."!  The  means  are  of 
two  sorts— first,  the  continuity  of  substance  from  parent  to 

*  Shull,  G.  H.  A  quantitative  study  of  variation  in  bracts,  rays,  and 
disk  florets  of  Aster  Shortii  etc.  Arner.  Naturalist,  vol.  36,  1902. 

t  Strasburger.  E.    f  ber  Befnichtung.    Bot.  Zeitung.  1901. 

J  See  Ritter.  W.  E.  The  power  and  methods  of  heredity.  University 
Chronicle  vol.  III..  Berkeley  Cal.  1900. 


280  PLANT  PHYSIOLOGY 

offspring,  and  second,  the  continuity  of  influences  to  which 
organisms  are  exposed.  These  means  are  continually  em- 
phasized and  for  the  most  part  ignored,  respectively.  The 
former  are  for  morphology  to  describe,  the  latter  are  within 
the  province  of  physiology  to  discuss.  Granting  the  con- 
tinuity of  substance,  which  we  have  already  considered 
(pp.  255-63),  what  are  the  influences  which  are  continu- 
ous? 

The  environment  of  an  organism  is  made  up  of  many 
factors,  some  great,  some  small,  as  judged  by  their  visible 
effects,  some  continuous,  some  constant,  some  variable, 
some  occasional,  some  periodic.  Among  continuous  factors 
the  following  may  be  mentioned.  First,  the  atmosphere,  the 
composition  and  pressure  of  which  are  unchanging.  Second, 
water,  which  has  the  same  composition,  the  same  solvent 
power,  and  the  same  carrying  power  always.  Third,  grav- 
ity, a  force  of  uniform  strength.  Fourth,  the  earth  as  a 
whole,  the  character  of  which  changes  only  with  inconceiv- 
able slowness.  If  these  factors  are  not  absolutely  continu- 
ous, taking  infinite  time  into  account,  they  are  continuous 
and  unchanging  so  far  as  millions  of  generations  of  organ- 
isms are  concerned.  They  are  exactly  the  same  for  the 
offspring  as  for  its  parents. 

On  the  other  hand,  light  and  heat  are  forces  which  act 
periodically  with  great  regularity,  but  which  are  constant 
only  within  rather  wide  limits.  Of  the  influences  which  are 
occasional,  other  and  motile  living  organisms  are  perhaps 
the  most  striking. 

The  continuous  and  the  periodic  influences  are  the  ones 
which  have  received  least  attention  from  those  interested  in 
the  problem  of  heredity.  These  influences  must  conserve  if 
the  variable  and  occasional  influences  introduce  differences. 
But  as  a  basis  for  all  conceptions  of  heredity  and  variation, 
we  must  concede  the  irritability  of  living  protoplasm. 

The  egg,  which  is  a  part  of  the  living  substance  of  the 
mother,  is  penetrated  by  the  spermatozoid,  a  part  of  the 
living  substance  of  the  father,  and  these  two  fuse  into  one 
mass  of  living  substance.  Each  particle  of  living  substance, 
while  still  within  the  body  of  the  parent,  was  influenced  by 


REPRODUCTION  281 

all  the  forces,  constant  and  otherwise,  which  were  then 
operating.  Assuming  the  irritability  of  living  protoplasm, 
any  force  which  influences  it  will  set  up  a  reaction  of  some 
sort  in  it,  though  this  reaction  may  not  be  visible  at  the 
time.  Between  the  time  of  leaving  the  parent  and  meeting 
and  fusing  with  its  mate,  each  sexual  element  was  influenced 
by  and  reacted  to  all  the  forces  then  operating.  By  the 
fusion  of  these  two  living  masses  into  one,  the  resultant 
effects  of  all  preceding  influences  upon  the  two  separate 
masses  are  united.  From  the  moment  of  fusion,  the  single 
mass  is  influenced  by  all  the  forces  and  matters,  continuous, 
constant,  and  variable,  which  constitute  its  environment. 
If  the  fertilized  egg  remain  within  the  body  of  the  parent,  it 
is  subjected,  during  the  period  of  gestation,  to  mechanical 
and  other  influences,  some  of  which  absolutely  limit  it  in 
form  and  size.  For  instance,  the  young  zygospore  of  Spiro- 
gyra,  the  young  oospore  of  (Edogonium,  the  foetal  colt,  in 
the  body  of  the  mother,  are  limited  as  to  form  and  size  by 
the  enclosing  walls  of  cell  or  uterus.  They  could  not  grow 
beyond  a  certain  size,  though  they  may  never  reach  this 
size,  because  growth  would  be  mechanically  hindered  and 
stopped  by  the  surrounding  cell  or  uterine  walls.  In  form 
also  they  are  similarly  limited,  not  as  the  mould  fixes 
the  size  and  shape  of  the  cast,  but  because,  by  excess- 
ive growth  in  one  direction  or  part,  they  would  encounter 
the  mechanical  resistance  of  the  enclosing  walls  of  cell,  or 
uterus. 

The  young  and  forming  individual,  whether  one-celled 
spore  or  many-celled  embryo,  is  affected  also  by  food, 
warmth,  position  with  relation  to  gravity,  etc.  Of  these 
influences,  we  know  least  about  the  most  constant.  The 
most  constant  influences  have  so  far  baffled  the  experi- 
menter to  eliminate.  Since  they  are  the  most  difficult  to 
eliminate,  it  is  reasonable  to  conclude  that  they  are  also  the 
most  powerful.  If  they  oppose  the  experimenter,  they  must 
also  affect  the  young  and  forming  individual. 

If  the  fertilized  egg  is  not  enclosed  within  the  body  of  the 
mother,  it  too  is  subjected  to  all  the  influences  composing 
its  environment,  to  influences  which  have  changed  with  in- 


282  PLANT  PHYSIOLOGY 

conceivable  slowness  if  at  all,  as  well  as  to  changing  influ- 
ences. 

The  dominant  power  of  the  unchanging  influences  is  shown 
by  man's  inability  to  make  fundamental  changes  in  living 
organisms  by  experiment,  and  by  the  absence  of  any  proof 
of  the  *  so-called  "inheritance  of  acquired  characters."  If 
man  could  eliminate  the  force  of  gravity  in  experimenting 
on  an  organism,  he  might  obtain  other  results  than  now, 
when  all  he  can  do  is  to  expose  the  organism  on  all  sides  to 
the  force,  or  to  oppose  gravity  by  centrifugal  or  other  force. 
If  he  could  change  the  composition  of  water,  or  find  a  sub- 
stitute for  it,  he  might  make  some  fundamental  change  in 
the  organism  under  experiment.  He  cannot  remove  the  air 
from  around  a  plant  or  animal  without  at  once  changing 
the  conditions  of  his  experiment  in  other  ways  also,  and  the 
result  of  such  experiment  is  of  little  value  in  the  subject 
under  discussion,  because  it  is  produced  by  a  combination 
of  changes. 

We  may  safely  conclude,  then,  that  the  unchanging  factors 
in  its  environment,  to  which  it  irritably  responds,  are 
among  the  most  powerful,  if  they  are  not  the  only,  influ- 
ences which  make  the  offspring  like  its  parents.  The  chang- 
ing factors  cause  it  to  vary.  Heredity  is  possible  only 
because  of  the  irritability  and  continuity  of  living  proto- 
plasm, and  the  continuity  of  certain  influences  acting  upon 
this. 

Heredity  is  a  mystery,  but  so  is  the  form  of  a  crystal  of 
common  salt.  Spirogyra  plants  are  no  more  and  no  less 
alike  than  are  crystals  of  common  salt.  Does  common  salt 
" inherit77  its  crystalline  form?  Crystals  of  common  salt 
represent  the  reaction  of  NaCl  molecules  to  their  environ- 
ment, of  which  some  factors  are  constant  and  others  chang- 
ing. Spirogyra  cells  represent  the  reaction  of  the  molecules 
and  combination  of  molecules  of  which  its  living  protoplasm 
consists,  to  their  environment,  of  which  some  factors  are 
constant  and  others  changing.  Given  the  continuity  of  the 
irritable  substance  (protoplasm)  from  parent  to  offspring, 
heredity  and  variation  are  the  inevitable  results  of  the 
constant  and  of  the  changing  factors  respectively,  which, 


REPRODUCTION 


283 


taken  together,  compose  the  environment.  These  influences 
at  least  we  can  more  or  less  definitely  study  in  their  rela- 
tions to  heredity.  Though  there  doubtless  are  other  factors 
contributing  to  heredity,  they  are  not  yet  included  within 
the  domain  of  physiology  and  need  not  be  discussed  here. 


INDEX. 


Absorption,  Chapter  IV.,  103-161 

Absorption    bands,    see    Chlorophyll 

Aerenchyma,  144 

Aerobic  respiration,   see  Respiration 

Age  of  cells,  123 

Air  passages,  143;  variation  in  size, 
142,  143 

Air  space,  proportion,  155 

Alcohol,  see  Fermentation,  Intramo- 
lecular Respiration,  and  Respir- 
ation 

Alpine  plants,  brilliancy  of  flowers, 
*  274 

Aluminum,  95 

Amides,  occurrence,  70;  as  waste 
products,  70,71 ;  as  stages  in  pro- 
teid  synthesis,  71 

Anaerobic  respiration,  see  Intramolec- 
ular respiration 

Animals,  differences  from  plants,  1 ; 
resemblances,  261 

Annual  rings,  123;  causes  of  forma- 
tion, 191-196 ;  more  than  one  ring 
per  annum,  195 

Antherozoids,    chemotaxis,  235 

AKCEUTHOBITJM,  90 

Arctic  plants,  brilliancy  of  flowers, 
274 

Ascent  of  water,  119;  Sachs's  hypo- 
thesis, 119,  121;  Godlewski's, 
119,  121,  122;  Strasburger's,  120, 
122,  123;  sap-pressure  hypothe- 
sis, 120,  127,  131,  133;  gas  press- 
ure, 120;  Jamin's  chains,  120 

Ash  constituents,  92+;  minimum 
amounts,  101,  102 


Associated  organisms,  252.  See  also 
root  tubercle  plants,  72-j- ;  humus 
plants,  78+ ;  carnivorous  plants, 
81+;  parasites,  85+;  lichens,  91, 
92 

Autumn  wood,  123;  causes  of  forma- 
tion, 194 

Auxanometers,  177 

B 

Bacteria,  respiration  of,  20,  21 ;  in 
fermentations,  30-37;  Engel- 
mann's  bacteria  method,  56;  nitri- 
fying, 68;  in  root  tubercles,  72- 
76 ;  N-fixing,  76 ;  associated  with 
carnivorous  plants,  83;  sulphur 
bacteria,  20,  98;  iron  bacteria, 
20 ;  nitrite  and  nitrate  bacteria,  20 

Bacteroids,  74 

Bleeding,  127,  132 ;  influence  of  tem- 
perature, 134 ;  pressure  in  bleed- 
ing, 134,  135;  amounts,  136 

Breathing  pores,  see  Stomata 

BRUGMANSIA,  90 


Calcium,  99,100;  Loew's  hypothesis, 
100 

Calories,  liberated  in  respiration,  fer- 
mentation, intramolecular  respir- 
ation, see  these  topics 

Carbon,  source  of,  43;  percentage  in 
air,  44;  quantity  in  air,  44 

Carbon  dioxide,  rate  of  absorption, 
45;  means  of  maintaining  sup- 
ply, 45,  46 ;  change  in  percentage 
in  air,  46 ;  effect  of  increased  per- 
285 


286 


INDEX. 


centage,  46;  principles  of  diffu- 
sion, 47;  aeration  of  the  plant 
body,  47,  48,  diffusion  through 
cell  wall,  48,  49;  stomata,  49; 
Stahl's  test,  49;  conditions  for 
absorption,  50;  for  elaboration, 
50 ;  rate  of  diffusion,  50 

Carnivorous  plants,  81;  bacteria  as- 
sociated with,  83 ;  enzymes,  83 

Carotin,  52 

Cell-division,  relation  to  growth,  165; 
relation  to  gravity,  198,  199 

Cell-sap,  active  agent  in  absorption, 
106;  density  and  composition, 
how  maintained,  111,  112 

Chasmogamy,  274,  275 

Chemical  fertilization,  230,  231,  260 

Chemical  stimuli,  226,  237 ;  conditions 
of  influence,  227 

Chemotaxis,  233-237 

Chemotropism,  231-233,  236,  237,  245 

Chlorophyll,  location,  51;  physical 
characteristics,  51;  mixture  of 
pigments,  51;  associated  pig- 
ments, 51;  carotin,  52;  solvents, 
52;  fluorescence,  53;  spectrum, 
53 ;  absorption  bands,  53 ;  amount 
of  light  absorbed,  54;  efficiency 
of  chlorophyll  screen,  55;  pro- 
portion of  light  absorbed,  55; 
amount,  55;  dependence  upon 
iron,  57,  58,  101;  upon  light,  57; 
in  guard  ceils  of  stomata,  147 

Chlorophyll  grains  or  granules,  see 
Chromatophores 

Chloroplastids,  see  Chromatophores 

Chlorosis,  101 

Chloro  vaporization,  137 

Chromatophores,  body  and  pigment, 
57;  photosynthesis  in  isolated 
Chromatophores,  57;  position  af- 
fected by  light,  217,  218.  See  also 
Chlorophyll 

Circumnutation,  246 

Cleistogamy,  272,  274,  275 

Climate,  continental  and  island,  220, 
221 


Colloids,  129 

Colored  screens,  56 

Common  salt,  93,  227 

Comparison  of  photosynthetic  and 
respiratory  activities,  65 

Conditions  essential  to  life,  6 

Conducting  tissues,  156,  157 

Contact,  irritation  by,  239-251;  ef- 
fect 011  amount  of  growth,  240, 
241 

Correlation,  251,  252 

Corrosive  action  of  roots,  125 

Crystalloids,  129 

CUSCUTA,  88-90..  244-246 

Cytoplasm,  ratio  between,  and  nu- 
cleus, 181 

Cytoplasmic  membranes,  106 

D 

Dew,  128 

Diastase,  64 

Diffusion,  principles  of,  47;  rate  of 
CO2  diffusion  through  minute 
openings,  50 ;  physics  of,  108 

Dissociated  atoms,  effect  of,  228,  229 

Dodder,  88-90,  244-246 

DROSERA,  82,  83 

E 

Ecology,  252,  253 

Ectoplast,  106 

Electricity,  237-239;  influence  on 
amount  of  growth,  238;  on  ger- 
mination, 239;  on  direction  of 
growth,  239;  on  locomotion,  239 

Energy,  need  of,  12 

Engelmann's  bacteria  method,  56 

Enzymes,  formed  in  connection  with 
respiration,  19,  29,  30,  33;  zy- 
mase,  33;  diastase,  64;  effect  of 
light  on,  209,  210;  in  carnivorous 
plants,  83  , 

EQUISETUH,  reproduction,  276 

Excretion,  by  roots,  125;  of  water, 
126-128 

Exercise,  advantage  of,  191 


INDEX. 


287 


"  Fatness  "  of  leaves,  64 

Fermentation,  definition,  30;  distinc- 
tion from  decay  and  disease,  30 ; 
alcoholic,  31;  by-products  in  al- 
coholic, 31 ;  zymase  in  alcoholic, 
33 ;  amount  of  alcohol  formed  by 
fungi,  31,  33,  34;  lactic,  34;  bu- 
tyric, 34;  oxidizing,  34,  35;  ace- 
tic, 35;  disease,  35;  diphtheria, 
35;  mixed,  36 

Fermentations,  30-37 

Fertilization,  essential  feature  of,  259; 
advantage,  260,  261;  processes 
succeeding,  188;  chemical,  230, 
231,  260 

Fischer's  hypothesis,  62 

Flower-forming  substances,  273 

Food,  influence  on  reproduction,  265, 
266,  273,  274 

Food  distribution,  Chapter  IV.,  103- 
161 ;  translation,  63,  155+ 

Food  manufacture,  see  Photosynthe- 
sis, Nitrogenous  foods 

Force  exerted  in  growth,  174-176;  in 
geotropic  bending,  208;  by  tur- 
gor,  110,  111,  130,  131 

Formic  aldehyd  hypothesis,  61;  Spi- 
rogyra  cultures  as  a  test,  61 


Gall-insects,     effect    on    growth    of 

wood,  195 

Galvanotaxis,  see  Electricity 
Galvanotropism,  see  Electricity 
Gases,  movement,  103,  104 ;  exchange 
through  stomata,  142 ;  rate  of  dif- 
fusion through  stomata,  50 ;  rate 
through    epidermal    cells,    143; 
movement,  151+ ;  pressures  with- 
in plant,  151-154;  composition  of 
enclosed  gases,  152-154;  propor- 
tion of  air  space  in  plants,  155 
Geotropism,  definition,  198;  of  roots, 
198-206;    sensitive    region,    200, 
201;  region  of  response,  200;  la- 


tent period,  201 ;  transmission  of 
stimulus,  201,  203,  204;  nature 
of  stimulus,  201,  202;  effect  of 
stimulus  in  cell,  202 ;  position  of 
greatest  sensitiveness,  204;  rela- 
tion of  sensitive  parts  to  light, 
207,  214,  215;  force  exerted  in 
bending,  208;  comparison  with 
heliotropism,  215 

Germination,  9,  10;  influence  of  elec- 
tricity, 239;  influence  of  light, 
213;  influence  of  contact,  240, 
247 

Gravitation,  196-208;  opposed  by  air, 
water,  soil,  197;  effect  on  rate  of 
growth,  197 ;  effect  on  first  divi- 
sions of  egg,  198,  199 ;  immediate 
effects  in  sensitive  parts,  197 ;  ef- 
fect on  parts  of  different  specific 
gravity  in  cell,  202;  chemical 
changes  in  cell  in  response,  202. 
See  also  Geotropism 

Growth,  Chapter  V.,  pp.  162-182; 
effect  on  respiration,  38 ;  depend- 
ence upon  irritability,  162-164, 
183,  240,  etc. ;  definition,  164, 
165;  relation  of  cell  division, 
165;  stages,  166;  relation  of 
water,  167;  factors  making  pos- 
sible, 168;  time  of  most  rapid, 
169-171 ;  periodicity,  170,  171 ;  ir- 
regularity, 171 ;  relation  of  tur- 
gor,  171-173 ;  relation  of  mechan- 
ical restraint,  174,  187;  force 
exerted,  174-176;  measuring  in- 
struments, 177;  rates  of,  177- 
179;  maximum  size,  180-182;  pro- 
portion between  cytoplasm  and 
nucleus,  181 

H 

Halophytes,  94,  95;  resemblance  to 
Haustoria.  245;  xetrophytes,  95 

Heat,  one  of  the  conditions  of  life,  6 ; 
minimum,  optimum,  and  maxi- 
mum temperatures,  219,  220;  ef- 
fect on  rate  of  movements,  221, 


288 


INDEX. 


222 ;  on  direction  of  movements, 
221 ;  as  an  element  of  climate, 
220,  221 ;  liberated  in  respiration, 
fermentation,  intramolecular  res- 
piration, see  tliese  topics 

Heights  of  trees,  119 

Heliotaxis,  216.     See  Light 

Heliotropism,  213-215.  See  Light; 
comparison  with  geotropism, 
215 

Heredity,  279-283;  definition,  279; 
means,  279,  280;  environmental 
factors,  280;  prenatal  influences, 
281;  subsequent  influences,  281- 
283 

Hot  springs,  plants  of,  220 

Humus,  78;  humus  plants,  78;  asso- 
ciation with  other  plants,  78-80 

Hydathodes,  128 

HYDKODICTYON,  reproduction,  266- 
268 

Hydrotaxis,  226 

Hydrotropism,  224-226;  balance  be- 
tween, and  geotropism,  224.  See 
Water 

Hygroscopic  movements,  226 


Insectivorous  plants,  see  Carnivorous 
Intercellular  spaces,  142,  143 
Intramolecular   respiration,    16.     See 

Respiration 

Intramolecular  respiration,  16 ;  defini- 
tion, 25,  26 ;  inherent  in  all  organ- 
isms,   25;     duration    limited    in 
higher  plants,  25,  27;  substances 
concerned  in,    28;  products,  £8. 
See  Fermentation  and  Respiration 
Inuliu,  160,  161 
Iodine,  in  marine  plants,  113 
Iron,  101 ;  as  stimulant,  101 ;  relation 
to  chlorophyll  formation,  101;  in- 
crustations, 126 

Irritability,  Chapter  VI.,  183-253; 
possible  physical  reasons  for, 
184-186;  effect  on  growth,  186. 


See    Geotropism,     Heliotropism, 
Water  currents,  etc. 


Jamin's  chains,  120 


Latent  period,  201,  243 

Laticiferous  tubes,  159,  160 

Leguminous  plants,  growth  in  steril- 
ized and  unsterilized  soil,  75; 
seeds  of,  261.  See  Root-tuber- 
cles 

Lenticels,  144 

Leptom,  158,  159 

Leptomin,  159 

Leucoplastids,  160 

Lichens,  91,  92 

Life,  definition,  256;  essential  condi- 
tions, 6 ;  how  limited,  254-256 

Light,  one  factor  essential  to  life,  6; 
relation  to  food  manufacture,  50, 
58-63,  69,  70;  relation  to  chloro- 
phyll, 52-58;  component  rays,  56; 
values  of  different  rays,  57 ;  rela- 
tion to  chlorophyll  formation,  57 ; 
effect  on  organic  substances,  208- 
210;  effect  on  rate  of  growth, 
210-213;  on  form,  211;  on  perio- 
dicity of  growth,  211 ;  on  sub- 
mersed aquatics,  211-213;  on 
germination,  213;  on  direction  of 
growth,  213-215;  relation  to  geo- 
tropism, 214;  comparison  of  he- 
liotropism  and  geotropism,  215; 
effect  on  locomotion,  216;  effect 
on  position  of  cell  organs,  217, 
218;  comparison  of  heliotropism 
and  heliotaxis,  218,  219;  influence 
on  reproduction,  264-268,  271- 
276;  on  brilliancy  of  flowers,  274, 
275 

Lime  incrustations,  126 

Living,  definition,  4 

Locomotion,  influences  affecting,  see 
Chemotaxis,  Phototaxis,  etc. 


INDEX. 


289 


Magnesium,  100,  101 
Maple  sap,  132,  133,  136 
Mechanical  effect  of  water,  189-191 ; 

of  other  substances,  226 
Mechanical  force,  see  Force 
Mechanical  pull,  effect   on   growth, 

187,  188 
Mechanical      restraint,     effect      on 

growth,  174,  187 
Milk-tubes,  159,  160 
MIMOSA,  247-251 
MIMULUS,  reproduction,  270-273 
Mistletoe,  86-88 
Motor  zone,  in  roots,  201 ;  in  tendrils, 

242 
Movement,  of  gases,  103,  104,  142;  of 

water,  Chapter  IV.,  103-161 
Movements,  see  Growth,  Irritability, 

Mimosa,  etc. 
Mycorhiza,  79,  80 

N 

Nectaries,  126 

NEPENTHES,  84,  85 

Nitrates,  69 

Nitrifying  bacteria,  20,  68 

Nitrogen,  distribution  in  plant,  66; 
occurrence  in  nature,  67 ;  sources, 
67 ;  nitrifying  bacteria,  68 ;  N-fix- 
ing  bacteria,  75,  76;  cycle  of  N 
in  nature,  77 

Nitrogen  bacteria,  20,  68,  75,  76 

Nitrogenous  foods,  occurrence,  70; 
origin,  70;  manufacture,  70,  71; 
storage,  71,  72;  use,  71,  72.  See 
Amides,  Proteids 

Non -sexual  reproduction,  advantages 
277-279 

Nucleus,  relation  to  growth,  181 

Nutrient  solutions,  means  of  transfer, 
116-125 

Nutrition,  Chapter  III.,  40-102;  rela- 
tion to  respiration,  40,  41 ;  stages, 
41;  furnishes  material,  42;  food 
materials,  42 ;  essential  elements, 

TO 


42;  characteristic  element,  43. 
See  Photosynthesis,  Nitrates,  Ni- 
trogenous foods,  etc. 


Osmosis,  106,  108-110 

Osmotic  pressure,  110;  effect  of 
changes  in,  229,  230 

Oxidation,  conditions,  13,  14;  in  res- 
piration, 18-20;  of  NH,  com- 
pounds, 20;  of  H2S  and  S,  20;  of 
Fe  compounds,  20,  21.  See  Res- 
piration 

Oxygen,  optimum  percentage,  14; 
effect  of  excess,  15;  in  respira- 
tion, 18-20 


Parasites,   85-92;   Arceuthobium,  90; 

bacteria   in    root-tubercles,    76; 

Brugmansia,  90;  Cuscuta,  88-90; 

lichens,   91,    92;    PJwradendron, 

86-88;  Rqfflesia,  90;  Basoumow- 

stoa,  90;  Viscum,  86-88 
Parasitism,  85;  advantageous,  86 
Parthenogenesis,  259,  260 
Petioles,  irritable  by  contact,  244 
Phosphorus,  96,  97 
Photosynthesis,  58-66 
Phototaxis,  see  Light 
Phototropism,  see  Light 
Physiology,  aim,  2 
Plageo tropic  organs,  198 
Plants,  differences  from  animals,  1; 

resemblances,  261 
Poisons,  stimulating  and  other  effects, 

230 ;  action  of  dissociated  atoms, 

228,  229 

Potassium,  98,  99 
Protoplasm,  a  structure,  7 
Pulvinus,  247 

R 

RAFFLESIA,  90 

RAZOUMOWSKIA,  90 

Removal  of  manufactured  foods,  63 


290 


INDEX. 


Reproduction,  Chapter  VII.,  pp. 
254-283 ;  chief  end,  257,  258 ;  defi- 
nition, 257;  modes,  259;  sexual, 
259,  271,  272,  275-279;  non-sex- 
ual, 259,  277-279;  precautions  to 
secure,  261;  vegetative,  261-263; 
influence  of  environment,  263- 
276 ;  stimuli  determining  mode  in 
Vaucheria,  264-266,  275;  in  Hy- 
drodictyon,  266-268;  in  Stigeodo- 
nium,  268,  269;  influence  of  os- 
motic pressure,  268,  269;  stimuli 
affecting  reproduction  in  flower- 
ing plants,  269-275;  in  Mimulus 
Tilingi,  270-273;  in  Viola  odo- 
rata,  274,  275 ;  in  Equisetum,  276 ; 
in  Sequoia,  277 

Resin,  126 

Respiration,  Chapter  II.,  pp.  12-39; 
definition,  13;  rates  at  different 
times,  15;  regulated  by  proto- 
plasm, 16;  reduction  of,  16;  sus- 
pension, 16;  object,  16;  yield  in 
energy,  17,  22;  substances  con- 
cerned, 17,  18;  products,  18; 
characteristic  product,  21;  heat 
of  combustion  of  sugar,  22,  23-; 
heat  of  alcoholic  fermentation^ 
23,  24;  of  butyric,  24;  of  acetic, 
24;  of  combustion  of  alcohol,  24; 
relation  of  enzymes  to  respiration, 
19,  29,  30,  33 ;  optimum  temper- 
ature, etc.,  37;  effect  of  injuries, 
37,  38 ;  rate  in  relation  to  growth, 
etc.,  38;  ratio  of  O2  to  CO2,  38; 
variation  in  ratio,  38,  39 ;  amounts 
of  COa  given  off,  39;  effective- 
ness of  bacteria,  39;  summary, 
39.  See  Fermentation,  Intramo- 
lecular respiration 

Response  to  stimuli,  see  Geotropism, 
Irritability,  etc. 

Rheotaxis,  190 

Rheotropism,  189 

Roots,  113;  corrosive  action,  125; 
early  growth  in  spring,  194;  geo- 
tropism  of,  198-206 ;  heliotropism, 


214;  thermotropism,  221;  hydro- 
tropism,  224 ;  chemotropism,  231 ; 
galvanotropism,  239 ;  thigmotrop- 
ism,  247 

Root-hairs,  114,  115 
Root-pressure,  see  Sap-pressure 
Root-tubercles,  occurrence,  72;  struc- 
ture, 74;  contents,   74;  mode  of 
infection,  75;   bacteria  parasitic 
in,  76 ;  fix  free  N,  75 


Salt,  93,  227,  228 

Salt  plants,  see  Halophytes,  94,  95 

Sap,  composition  of  maple,  132 

Sap-flow,  132,  133,  136 

Sap-pressure,  distinction  from  turgor 
pressure,  131;  figures,  135,  136; 
relation  to  ascent  of  water,  120, 
127,  131,  133;  "root-pressure," 
135 

SAKHACENIA,  84 

Sea-water,  94,  95,  197 

Secretion,  125-130 

Seeds,  respiration  in  air-dry,  9;  dur- 
ation of  vitality,  10 

Selective  power  of  roots,  etc.,  112, 
113 

"Sensitive  plant,"  see  MIMOSA 

SEQUOIA,   reproduction,  277 

Sexual  reproduction,  see  Reproduc- 
tion;  necessary?,  276-279;  in  in- 
fusoria, 279;  in  diatoms,  279 

Sieve-tubes,  157-159 

Silica,  93 

Soil,  comparison  of,  in  Eastern  and 
Western  States,  11 

Soil  water,  104,  105 

Span  of  life,  how  limited,  254-256 

"Spring  wood,"  123,  191-194 

Staining  living  protoplasm,  107 

Stamens,  sensitive  to  contact,  251 

Starch,  proportional  and  structural 
formulae,  59;  possible  mode  of 
formation,  61,  62;  translocation, 
63,  64,  99;  in  sieve-tubes,  158; 
storage,  160 


INDEX. 


291 


Stems,  geotropism  of,  306,  207 

St  reotropism,  see  Thigmotropism 

STIGEOCLOXIUM,  reproduction,  268, 
269 

Stigma,  secretion  on,  129;  sensitive 
to  contact,  251 

Stimulants,  230 

Stomata,  49,  142;  structure,  145; 
mechanism  of  opening  and  clos- 
ing, 145-147 ;  function  of  auxiliary 
cells,  145,  148;  size,  149;  propor- 
tion to  leaf  surface,  149;  condi- 
tions of  opening  and  closing,  148- 
150;  with  fixed  guard  cells,  151 

Storage  of  foods,  160 

Sulphur,  97,  98 

Sulphur-bacteria,  20,  98 

Sunlight,  see  Light 


Temperature,  fatal,  8,  9 

Tendrils,  241-244 

Thermotaxis,  221 

Thermotropism,  221 

Thigmotropism,  247 

Tides,  effect  on  growth,  190,  191 

Tonoplast,  106 

Toxic  substances,  see  Poisons 

Traction,  iufluenceson  growth,  187, 188 

Transfer  of  nutrient  solutions,  116-125 

Translocation  of  foods,  63,  155,  156 

Transpiration,  136-141 ;  conditions, 
136, 137;  difference  from  evapora- 
tion, 137 ;  means  of  reducing,  137, 
138;  means  of  increasing,  138;  in 
moist  tropics,  138-140;  move- 
ments increasing,  142 

Traumatropism,  247 

Trees,  heights,  119 

Tropical  plants,  transpiration  in,  138- 
140 

Turgor,  definition,  111;  relation  of 
potassium  salts,  99;  relation  to 


growth,  171-173 
Twining  plants,  245,  246 


U 


UTRICULAKIA,  85 


Vascular  bundles,  117 

VAUCHERIA,    reproduction,    264-266, 

275 

VIOLA,  reproduction,  274,  275 
VISCUM,  86-88 
Vitality,  suspended,  10 

W 

Water,  vehicle  of  food  materials,  6; 
essential  component  of  active 
protoplasm,  6-8 ;  in  air-dry  seeds, 
8;  in  soil,  104,  105;  conducting 
tissues,  117,  118;  ascent  of,  119- 
123, 127,  131,  133;  storing  tissues, 
124;  secretion,  126-128;  pores, 
128;  glands,  128;  flow,  132,  133, 
136;  relation  to  growth,  167;  ef- 
fect on  growth,  222-226 

Water-currents,  effect  on  growth,  189, 
190 

Waves,  effect  on  growth,  190,  191 

Weber's  law,  236 

Weeping,  132,  134-136 

Winds,  189,  226 

Winter-killing,  150 


Xerophytes,  95 


Zinc,  95 

"Zinc  soil  flora,"  95 

Zoospores,  influence  of  contact,  240, 

247 
Zymase,  33 


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